Well, that's not "quite" true. At most conferences I've been to, the "peer review" happens in real time with questions from the audience after the talk. In some cases, those "reviews" have been pretty damned scathing.
You make a good point, but the abstract is still unrefereed in the proceedings. Also, I've seen many graduate students, presenting for the first time, getting blistering attacks from some grumpy professor with a hot pepper up his @ss. At that point, the student's advisor, or more sympathetic audience member, will step in and correct the noise maker. At other conferences, I've seen industrial work criticized by an academics as to being not possible or goes against such and such principle. The author simply retorts that the process makes a selling product in wide use. If the process was impossible, how can we have a finished product. Some professors are out of touch with what happens in the real world, even in science.