(1) nowadays, a lot of snapshots wind up on the web. Until ~10 years ago, people could assume that in the worst case they might wind up in some magazine. But now, regardless of the quality of the image and the professionalism with which it was made, millions might see that image.
(2) some folks are also reacting quite strongly to "big brother" fears and there are cases where this is warranted. For example, some government employees worry about being recorded in the context of a street protest or such because that puts them automatically on a watchlist which could then delay (or derail) promotions. It used to be unlikely that this would happen, but now, with the possibility of facial recognition software in use even at big sporting events....
(3) the number of people who will point a digital camera (or cell phone) at a given street scene has gone through the roof. Nowadays anything of interest will be surrounded by large numbers of people recording it with all kinds of gizmos. If you happen to be the subject, then of course this could be quite intimidating. I think we've seen the effect on government officials as well, there is the fear that somebody's gonna 'capture' you, and then it'll wind up, photoshopped, in some weekly grocery rag. In this context it is a lot more reasonable for people to resent street photographers than it was 10+ years ago.
Overall, discreteness is just as important for street as it is for wildlife... we want to record the subject in a natural disposition. This is my issue with Winogrand, so much of his work (esp. later work) was borderline invasive IMHO. And if somebody sees your camera rather than your face, then of course they will react aversely.
In the end, every photographer has to assess the effect of being seen by the subject. Some may prefer it, citing emotional connection; others may prefer the wildlife-stalking (for lack of a better term) discrete approach. It's definitely a good thing that we don't all think the same way and take the same photographs.