No, if it hasn't been tested by the FCC for that purpose then it can't be sold for it. Simple as that.
If they didn't buy the driver to use the radio, then it won't work.
And most importantly, if the radio is not connected to an antenna, it isn't going to work.
If the seller of the phone believes 90% of the people aren't going to use the FM chip, then they can save lots of money by not having it tested by the FCC, not developing software/buying drivers, and not hooking up the hardware internally.
Even if as you say it's the same exact phone as in Europe, and they have the license for the drivers... they may just not get it tested by the FCC. Then no matter what, it's going to be illegal to activate that radio if it was never tested by the FCC.