There is a certain anti-authority edge to most of the good guys in the series. I think this plays, slightly, to the underdog card again, because we all pull for someone who's taking on the establishment. This is especially the case with Umbridge and Filtch. We scoff at these rules, as most of them are silly and useless anyway, and everyone can identify with the students against these two. No one wants to be controlled at the level that these two aim for.

I always read McGonnagall as a strict teacher type, not allowing much by way of rule breaking. She gives the trio and Malfoy detention in this one, for being out of bed wandering around. She has her moments were she lets things slide (Harry's broom riding = seeker) and definitely tends to undermine Umbridge.

Dumbledore seems to be more of a break with convention kind of guy, coupled with a learn by experience mentality. He's so in tune with what these kids are doing that he only steps in when it appears one will be really hurt (up until Goblet of Fire anyway). He can't be everywhere at once, right? He did stop Harry from visiting the Mirror of Erised. He breaks with convention on the anti-discrimination stance (Hagrid, Lupin, Dobby, Firenze) and refusing to submit to the Ministry in Order of the Phoenix.

I don't think Dumbledore or McGonnagall want any student to be put in harm’s way, hence the rules to begin with. Dumbledore seems much more tolerant (and sometimes encouraging) in my mind though, especially if he anticipates a positive outcome. The ends justify the means, in his mind, as with saving Sirius and Buckbeek.