I haven't seen a smartphone with onboard FM hardware in a while. They aren't simply "disabling" it - an FM receiver costs more, requires board real estate, and as you said, has the additional challenge of an antenna.
It's cost for a feature very few people use. FM is deprecated and obsolete - it's been dead in Europe in favor of DAB for years, and in the USA, satellite radio is the go-to for vehicles and streaming is the go-to for anywhere with wired Internet access (the backhaul for wi-fi in 95%+ of cases is wired DSL, cable, or fiber).
The NAB should look at themselves before complaining about others. FM is no longer a desirable feature for most people thanks to Clear Channel abusing every loophole in station ownership rules (There are various rules that are supposed to prevent one company from owning too many stations, among other things to promote a diversity of content.) The end result is that the content of FM stations is utter crap. The last time I drive without XM, on a single 4-hour drive I listened to one song at least three times, I think it was four. There were numerous other repeats. Meanwhile, if I do that drive with my XM subscription, it's rare that I'll hear even a single repeat.
Simply put, if a phone has FM now, I see that as a reason NOT to buy it, because that is paying extra for hardware that I'm NEVER going to use.