What it generally boils down to in trademark enforcement is specific knowledge. If you specifically find out somebody is violating your trademark you have to go after them, if you don’t know about the specific you can’t be expected to enforce. Which is the problem of the internet age, before the internet every other town, church and club in the country could hold a Super Bowl party and the league could reasonably be expected to not hear about any of them because NYC based operations didn’t tend to hear much about the goings on in Sheboygan Wisconsin. Now in the internet age there’s no such thing as local news or events, if one guy in the league office has a story about some party hit his news feed there’s now multiple records that the league “knew” about the party. They no longer have the ability to say “we didn’t know”, which gives them the obligation to enforce.
I guess you’re not old enough to remember all the national ad campaigns with Super Bowl promos for which the NFL did nothing to stop. When they started charging for official NFL sponsorship the companies ponying up started to complain. So as usual the NFL gets to have it both ways. When it’s to their benefit they do nothing. When it’s in their benefit to act, they “vigorously defend their trademark.” Just please don’t give us the pious baloney about trademark laws.