This is a logical fallacy. Let's say you play... Football/Soccer (or pick your favourite sport) - the equivalent would be saying that only Spain and Germany should be allowed to complain about bad calls by the refs. Every other professional team, every university team, every kids team, and every for-fun game in the park all aren't good enough (in your eyes) to complain about an unfair game.
In reality, if the ref is the mother of one of your opponents, and she's clearly making biased calls - a 6 year old can see that and legitimately complain despite that the 6 year old is not Mesut Ozil. Just because it's a game in university, and they aren't pro - doesn't mean their desire to play a fair game, or their desire to win - is any less valid.
A shadowpriest can legitimately feel they are struggling with single target, come to the forums for help, talk directly to many of the best spriests in the world (not something footballers get to do) - and discover that, 'here is what you can improve, but you are right to be concerned to some degree. The Refs decided all Spanish players would grow 20% longer legs this season'.
People's concerns, informed by facts, are perfectly valid to express - without you attacking their raid progression. Topping the meter in single target isn't what anyone is asking for, right now we're bottoming the meter (ST) - and that's not fair either - given we don't top the meter on any style of fight. Since we're apparently not meant to compete with Locks for multi-dot, it follows that we should not suck at single target.
For example:
Iron Juggernaut is a single target fight, it's the closest thing to a Patchwerk this tier - note that Affliction is 60k above us, tied for second place (this has been a consistent trend all expansion). Shadow is the lowest top spec of any given class.
Some locks are below us, but any lock who cares about beating Iron Juggernaut is Affliction - which says those locks are either not trying, not informed, or both. Ditto for Hunters who do the fight Marksmen, when they could do 70k more by swapping to Beast Master. The only spec below us who could give legitimate complaint is Ele Shamans - because they can't easily swap Enhancement - and their complaint is with bad balance just as is ours - it's not a complaint against Shadow's contention of being bad at Single Target. Further, by exploring those logs a little more, the top Ele's are pulling 330k+ DPS, but quickly fall well below that - which suggests that Ele Shamans just aren't being brought to raids much by top guilds (access to BIS gear).
Edit: For example, here's General Nazgrim - probably our best fight this tier - Shadow does pretty well on it via multi-dotting, Cascade, and near-permanent ToF uptime (plus it's a damage buffed fight, which always multiplies off ToF for an inordinate Shadow bonus). We do pretty well on it, but to spot the Affliction Locks - you'll need to look 20k above us.
Interestingly, you'll also notice that as raids gear up, and mobs therefore die faster, Shadow's ToF uptime is dropping - so while all the other lines are gaining DPS each week, Shadow's plateau'd and is now beginning to actually lose DPS despite rising gear levels. So, as the tier progresses - Shadow even on our best encounters are being automatically nerfed by the gear increases of our raids - faster than the benefit of our own gear increases is improving us. I think that's also a perfectly legitimate complaint.