It seems interesting that they did this after the TREMENDOUS debate about "PVP for PVE content" with prior legendaries, ie the ring or the cloak (I can't remember which; maybe both?), and I'm pretty sure I recall them saying they wouldn't do it again.
Now, to be fair, this is a hidden appearance for the entire spec and it technically isn't PVE-exclusive. It has no PVE requirements in terms of raids or anything else and anybody can start/complete the challenge. On the other hand, though, the scenario is a pretty solid example of PVE content and in that sense, also as a technicality, you are obtaining the appearance specifically through PVE (which is kind of unfair to say, as anything that's not 'you fight another player' is PVE and there's no other way they could go about it).
At the end of the day, anybody can do the scenario and unlock the weapon - the only real advantage PVE players have over PVP is that they're more likely to have better gear. There are a *lot* more players sitting with an overall 890-905 item level through PVE gear than there are through PVP, and that's because PVE gear is just flat-out more abundant (and, some would say, is easier to obtain with higher item levels). Aside from that, though, there is no inherent advantage given to PVE players over PVP. You can't even really argue a skill or content-familiarity advantage; while the average PVE-only player would probably do terrible and get destroyed if they stepped foot into a relatively high-level arena match, a PVPer stepping foot into the scenario (which is our PVE equiv of a higher-level arena game) isn't going to have trouble adapting; reacting to things going on around you and adapting your strategy is something they do all day long and I highly doubt it's going to take long for them to master the oh-so-difficult PVE mechanics presented to them there.
Anyways, I'm rambling - there's been a debate regarding PVP content requirements for PVE rewards for a long time, but you can't really call this an exclusively PVE reward. There is no PVE requirement needed to obtain it and it is a reward for the spec as a whole. You could argue that PVPers already have "their" artifact appearance as one is a reward from Honor Prestige levels... but then you could also argue that PVEers have "their" artifact appearance through literally every other available option. I think one variant of the hidden artifact being available to PVPers is fair, it just sucks that it's such a cool color.
Let's look at it from their perspective; you see and fall in love with the "Flame Glaive" appearance, but there isn't the RBG-related variant for it. You have no desire to do PVE content and honestly don't really have the time for it; you don't raid because the limited time you're able to spend on WoW is used for arenas and RBGs and such. You already have to complete a challenge scenario that is pretty much the DEFINITION of "PVE" just to get the default appearance, but can now
never unlock another color for it by playing the game the way you want to play it. Think about how the other appearances look (in terms of methods of obtainment) to a PVPer; having to kill EVERY SINGLE dungeon boss? Having to kill the end-boss of the next raid tier on something higher than LFR or Normal? These are things that they dislike doing JUST as much as you dislike participating in group PVP, except that EVERY SINGLE option requires it.
PVP isn't the "side quest" of WoW anymore. WoW isn't as much a "PVE game that also has PVP on the side" like it was years ago. Playing solely for PVP is something many players do at this point and is a normal, accepted avenue of gameplay. They have their own talent trees and now they literally have their own entirely unique stat sheets for PVP-specific balancing. It's an integral part of the game at this point and it's pretty reasonable to let them have *one* color for the appearance that they can obtain by playing the game the way they want to play it.
But, there was a much better way Blizzard could have gone about this.
The problem with Blizzard is that they make the same mistake time and time again; putting up this invisible barrier between PVE and PVP with things like this, elements of the game (aesthetic or not) that say "this is for you if you PVP, this is for you if you don't". It isolates people from portions of content and basically says "if you happen to want something the other guys get, you've got to do something you don't enjoy doing" -- which, in a
game, is the worst thing a player could ever hear; they're playing to have FUN! They want to do the things that THEY want to do.
All they had to do was give appearances dual-requirements, or at least give it to SOME of them. Why couldn't they simply have an appearance that said:
Purple Flame Glaive Variant
"Defeat every Legion dungeon boss *or* complete 10 rated battlegrounds while wearing this artifact appearance."
DONE. Problem solved. Give the purple variant another requirement instead of the RBG one. EVERYBODY is happy, and you can add PVP requirements to EVEN MORE colors to allow PVPers to obtain them without feeling like they're forced to do something they dislike (and also without PVEers complaining about having to PVP).
There are simple solutions to so many "problems" (I quote the word because this specific example is more "poor design habits") like this and it's always been totally confusing to me why Blizzard doesn't ever think of or implement them. I wish I had some sort of presidential emergency red phone that connected directly to the dev team, so I could pass tidbits of wisdom to them and prevent avoidable oversights like this.