Hasn't been true since 1.12.
http://us.battle.net/wow/en/status#type=pvp
Look at all those PvP servers that you're claiming don't exist. You know better.
The correct position is that they signed up for a PvP server, in which case the PvP ruleset applies.
And that they were never promised that they would only ever play with people from their server, and that this hasn't been the case in the game in practice since 1.12, increasingly so.
Creating a false dilemma doesn't mean your two options are the only ones. Especially when you're basing it on the false premise that because of cross-realm content, servers "don't exist". Which is just fundamentally untrue to even a cursory glance, and you know it.