I called MasterCard twice and both times they a) guessed that I was calling about content on Steam without ever mentioning Steam b) said that they only restricted "illegal adult content" and have "standards based on rule of law". Said absolutely nothing about protecting the brand. Also couldn't say if said "standards" were actual laws or MasterCard's own (legal) standards.
Good argument to record any calls with them and submit the recordings to the press
You don't really have to record the phone call. If anyone in the press wants to hear them saying that they don't block legal content, call them and ask about Steam. They have a ready-made PR response that they will read to you.