Exactly. When a show is riddled with F-bombs, you get desensitized to it. They mean nothing. But when in a PG-13 movie (where they only get one, non-sexual F-bomb) you get more mileage out of it. You can either play it for a big laugh or a powerful statement. In an R rated movie, its just another line and packs zero punch.
There is a line in the comic where Negan says "I'm not going to curse here just to show how serious I am." For his character, him NOT using the F-bomb had the same effect as others using one. I think his use of f-bombs comes partly from him not having the vocabulary to express himself better and to put up a front and make him look tougher than he is. Its certainly a big part of his character so it will be interesting to see how the show changes him. The Governor on the show and comic were quite different so I wouldn't be surprised if Negan is changed similarly. I actually felt the show's Governor was better than the Comic's version.
It is funny what we in America think is offensive. Graphic Violence is probably at the bottom. Then foul language. And finally sex and nudity. I think its practically reversed in the UK