At least nowadays many places bother to train TAs. My understanding is that not too long ago, the TA was just handed a syllabus and told to teach a class. Some schools had a reputation for admitting excess graduate students just to serve as TAs for a bit before being shown the door.

However, there are some universities that focus on quality undergraduate education. In those places, teaching ability is a big part of the hiring process and people have been denied tenure over poor teaching. It's the big research universities that have historically been lax in their teaching standards.