Lots of people compulsively select text when they read, and this feature makes that distracting.

That's an argument against the "helpful" little box that pops up when you select on NYTimes ... but that's not what this is about. This is silent (until you copy/paste anyway).

I can't speak for other browsers, but in Firefox there are some brief but distracting UI changes when you select text. It is a side-effect of the click tracking code they use.