I don't think this is quite the same, though, even if Twitter was preventing links to certain external sites from working outright. This is simply Twitter inser

I don't think this is quite the same, though, even if Twitter was preventing links to certain external sites from working outright. This is simply Twitter inserting some specific content between the link and the point when the target site finally loads. This is no different from if when you buy a newspaper at a stand the attendant says "by the way, you shouldn't read that newspaper, it sucks!" Is that allowable? What if the stands are actually a chain and all the attendants are instructed by corporate HQ to say that? IMO all these behaviors should be allowable. But, like I said, it's dumb. It's no different from doing a global server-side search-and-replace from "New York Times" to "New Poop Times". It's both transparent and childish.