Per stingraycharles (https://news.ycombinator.com/item?id=17709612 )'s comment, the problem is not so much the in-memory copy as the modification of that copy (although, of course, Javascript also modifies the in-memory copy of HTML … all these technical technicalities get in the way of the legal technicalities that Facebook wants to interpret as "we get to do what we want, and you don't get to do what we don't want").