It's a case of 'use at your own risk'.
Most services that have security in mind know VPNs exist. They might not be able to tell who/where you are, but they can definitely tell you're using a VPN. Just like using a PO box lets someone know you aren't receiving at your house.
Take these bits of info with a grain of salt, as I'm not the Hand That Moves of blizzard
-In a void, Blizzard has no problems with VPNs on their own. Dev's/CMs have said this.
-Blizzard specifically prohibits the use of VPNs to circumvent local law (WoW is not allowed to be played/connected to in some countries) and they will enforce this. It's Blizzard saying "Its them, not us, but we have to obey it"
-People have gotten banned for using VPNs legitimately
Use at your own risk. In theory it's fine. In theory even if you get banned, it should be fixable. None of that actually protects or prevents from getting banned and denied appeal.