What prevents me from broadcasting on 101 FM?
A radio station that already has the band, depending on where you're thinking of doing it, unless
the station hires you to broadcast. (From the look of it there are thousands of FM stations around
the country and elsewhere who have that band, but I don't think it's occupied in
every known
radio market.)
If you're in an area where no one's broadcasting on it, you could do it, assuming you have the
capital to build and operate a powerful FM radio station. Without the FCC, you'd have
a better chance of entering through a free market, assuming there aren't other regulatory hurdles
you'd need to clear. I think that's how it works, though I could be mistaken. (Sometimes stations
have a band with decimal point and advertise themselves up to the next non-decimal number; I
once worked for a pair of stations under the same ownership, one AM and one FM, and the FM
station's actual band was 103.9 but it advertised itself as "Cool 104.")