As infractions go, this isn't buying recruits or letting players live in mansions for free, granted. But it is a serious case of "where there's smoke, there's probably fire." Yes, sure, these are "technicalities" but remember what technicalities are: they're rules.
Even aside from the competitive advantage that sneaking practice hours gets you (in Michigan's case, I guess the answer to that appears to be "very little") we have to recognize that being a student athlete, especially in the modern era of graduation rate requirements, is a difficult job, and allowing coaches to take hours here and there just makes it worse.
If the NCAA's mission is to protect the student athlete, this is one of the most legitimate infractions cases they've pursued in quite some time because, in a very real sense, they're protecting athletes (at Michigan and everywhere else) from "practice creep" where the 4 hour-a-day limit morphs into four-and-a-half, and then into five, and the only option the athlete has is to just grin and bear it because you know there's some hot-shot freshman who's just dying to get on the field.
The bottom line is that the rules are the rules, and if the limit is four hours a day, a coach with competence and integrity would ensure that his workouts didn't exceed that limit.