This also goes back to the rumors of warning teams about the ball boys as well. Just because teams are very suspicious about Michigan's level of knowledge of their play calls and designs does not mean that teams had any idea at the level they were going.
People were suspicious of the level of information Michigan had about play calls and went and picked approaches that they thought were logical explanations. Just by looking through old-fashioned examples of gameday gamesmanship, teams know some teams push the rules to get an edge. So with MeCheaten, some thought it was sideline shenanigans, some thought it was old signs or playbooks being passed about, and it makes me wonder if some of the wild rumors teams blamed it on extra spotters in suites. All of those things are teams just trying to get a grip on a phenomenon that supposedly happens in a game in real-time and trying to come up with an in-game counter to stop this real-time activity.
None of it those types of issues are something that you can combat when the cheating is not happening in game in real-time but instead through a concerted effort to decode signs through earlier obtained sideline video and pairing with other data well before the game ever started.
I think teams are so angry about this because everyone was used to guarding their systems from an in-game real-time issue and so no one was really prepared for something of this level of sophistication. I think this is the biggest issue here and why now people have such a visceral reaction to the concept.