Idk, not sure why people are surprised.
Because there is an implied precedence of impartiality given the thousands of hours (and several years worth of) streaming from AT and other servers. Its just that Blizzard is butthurt this time around as some "efamous" people have moved over and snowballed a lot of lemmings with them. There is also an implied precedence of impartiality given that twitch.tv sponsors e.g. music talkshow and gamedev channels where the ownership and copyrights of the content are questionable and/or uncertifiable.
Because Blizzards rights to prosecution don't necessarily extend to streamers in the EU (yes I realize they have a presence in the EU but streaming rights are different there). This is why some people are encouraging a move to hitbox because its more of a "by the people for the people" place to stream.
Because certain terms and conditions that were in effect during Vanilla do not transcend into modern grounds for legal action. Basically every time there is a patch they have to force a new EULA and you have to sign it.
Since about 2008 Blizzard got smarter lawyers and slipped in redesigned EULA terms that basically make you "waive all rights". However, Vanilla client/server and all the Vanilla related IP/EULA content predate this modern war on freedom of expression/play. So it stands to reason they can only enforce so much (maybe they could enforce AT WOTLK and beyond...but certainly not Vanilla imo).
Because the cited reasons for the take-down/ban are based on the DMCA which was established in ~1996 and whether or not it extends several notions (such as the "fair use" clause) to streaming are still open for debate. Basically "fair use" means you're allowed to get dressed up like a full blown night elf and run around telling everyone you're Illidan, because of course when it suits Blizz, thats ok, but when you're showing people their game, you know, thats bad.
Because its clearly just a blatant scum move. They clearly can't take down the priv servers (not sure what the loopholes are but they've almost surely tried). So instead, they go after the fans and threaten Twitch (lets be clear twitch basically has to err on the side of caution until this is all sorted out -- warnings before bans would've been nice, though).
I'm sure there are more reasons to be surprised, shocked, and irrate insofar as feeling once again abused by these trolls. In my case it doesn't surprise me one bit (that it would be attempted) considering how rotten the company has become.