There is a clause at the end of the paragraph of the league rules (section covering this incident) that seems to be the contentious part of what the issue is all about - i.e. how it can be interpreted as it looks like it is clearly open to interpretation although the league's intention is that it not be.
On the one hand, the rule appears to be ambiguous, and I suspect that this is how Belichik interpreted it. For sure, technically an infraction of the rules, but open to interpretation nonetheless.
Interestingly, none of the articles reference the full paragraph (only in part as far as I have seen) regarding the specific rule in contention. Our local news station in Boston, however, had the full paragraph on last night's sportscast.
Note: stealing signals is not against the NFL rules - i.e. not cheating, but using video camera during game may be. And, the Pats got caught, due to Belichik's misinterpretation of the rule.
-- Tom
P.S. Can't find the full NFL rules online, only a digest which is incomplete.