IIRC the issue is not sign stealing per se, but attending opponents' games. The filming has nothing to do with it, apparently. Just going to the games at all is the violation. It's not clear if it's okay if a complete stranger (or at least someone not employed by your school) goes, videotapes the opponent, and donates the film to you.
This is a fairly new rule (or guideline or whatever they call it), from the 1990s I think. The original discussion was about fairness for the poorer schools, because of course those tickets and the travel are just beyond the means of Toledo, Miami of Ohio, and their ilk. Nothing about whether "sign stealing" is some sort of ethical or moral affront, mind you. There was talk of going in that direction so maybe something was added that makes it a foul for your staff to watch opponents' sidelines during a game and discern their signals and communicate their findings to coaches either live or at half-time.