That is pathetic. USB is an option the system is equipped with and is advertised. Doesn't matter if they like it or not.I've been having the music problems too. Mazda corporate said they aren't terribly concerned with the USB music problems i experience and said I should get used to Bluetooth. Odd for a car company, but I guess there ain't dollars in fixing something like that.
I use a USB drive because I find that Bluetooth barely works. I get far more functionality for browsing my music with USB.
Last time I tried USB it couldn't sort out my music from system files and made it a nightmare to sort through.