Inexperienced trainers tend to have a somewhat idealized view of how adults will behave in training. The expectation is that they will act like... well, act like adults. They'll pay attention, work hard, avoid fooling around, act and communicate with respect and so on. As a result, trainers will sometimes encounter situations where they are not prepared for, and fail to deal effectively with non-constructive behavior that comes from adults.
When you've been around adult learners in classroom situations long enough, you will end up having seen almost every kind of non-adult behavior imaginable, from reading newspapers during training, flying paper airplanes, insulting other learners, making passes at the instructor, and so on.
It's great to have positive expectations about the learners in a classroom, but the problem lies with not expecting and not preparing to deal with non-constructive behavior. It's best exemplified by this trainer quote:
I was so stunned when Steve said "insert offensive comment" that I didn't know what to do, and I didn't even believe I heard what I thought I heard".
Trainers need to deal with learners as they are and as they behave in the moment, and not rely on theory if it results in poor preparedness to deal with childish behavior.