Yes, the new feature is exactly like incorporating a feed reader, which is of course why I made that comparison. However, the purpose of Facebook -- or at least the purpose most users have for it -- is not to provide them and their acquaintances with instantaneous updates about each other's activities. It is a tool for social networking. A way of keeping in touch with your friends that is built in most ways like an extension of real life, not a replacement of real life with completely different features. When the features of the site get too far away from the ways in which information is shared in meatspace, people get uneasy because it's not the sort of social interaction that they have been living with for 20 years.
If my method of accessing the information changes, that *does* affect the other users, because it is *their* information. They are used to that information being delivered in certain ways based on the system that Facebook had set up. Because people care about the way information about them is presented, changing that system is going to upset people.
It doesn't matter *at all* that the information was already there. Everybody knows that the information was already there. It's about the *ways* in which people prefer to share certain types of information. Previously it was a way that most users thought felt natural, and now many users no longer feel that way. Maybe they'll change their minds after getting used to it, and maybe not.
The fact that third party tools already existed to do this is also not important, unless use of those tools was pretty widespread to the point that "a lot" of people (whatever that means to any individual user) already had this aggregation going on. As long as those users were a pretty small minority, it would have a very small impact on most others' use of the site.