Mostly what you said in that last line. There are lots of reasons for it, but AF culture has evolved to the point where, currently, absolute compliance with written instruction is more important than airmanship, judgment, or even common sense. In fact, it is even worse than that, because the "culture of conservative" even means that even something that is not against the rules, but is not explicitly authorized, trained to, and has explicit approval from the chain of command is treated the same way.
Knowing all of the parties involved, the flight lead got off light and the wingmen got hammered far beyond any actual culpability or deserve...all basically to "send a message".