Re: tumblr is not the government
Tumblr can (and does) remove, block and prohibit content *on it's service*, and should be able to do so. In fact, restricting tumblr's right to do so would be an infringement of free speech.
Sorta... IMHO it is much more about preserving Tumblr indemnity under Section 230 of the CDA and other statutes than about anything else. If it starts killing posts for openly political reasons it will lose its immunity under that section. Thus, it removes only content which has been declared an exception to that section in the CDA, one of the later statutes or as a precedent/ruling by a court.
If USA gets a law about censoring anything on the basis that it smells of the foreign enemy du jour they will do it with glee. As long as there is no such law, they will not threaten their CDA immunity so nothing will be deleted.