The Pats had a whole system, with personnel specifically filming and hiding the fact, others analyzing the video, another sending in the plays during the games, etc. Everything I've read said that it was illegal from the sidelines, but not from other areas.
The Pats never said that they thought it wasn't illegal. In fact they tried to hide that they were filming and had lines to give to security saying they were filming for other reasons, implying they knew filming from the sidelines wasn't allowed. Belichick's excuse was along the lines of "i misinterpreted the rules and didn't know that it was illegal to use the info in a live game." Anyway, if it's allowed from one place, why not another? And, before filming plenty of teams simply had specific people watching and writing down signals, filming just made it more accurate/efficient. Also, they were just as good if not better after spygate and after the NFL put radio in a defense helmet. But then again why would they spend that much effort for 40+ games to film and analyze? IMO, it's similar to deflategate, they found a way to exploit the system and did. Other teams did both as well, the Patriots were just better at it.
I still think it's a piece about Goodell, overall, like I said in my previous post.