Yes it is quite easy, that’s how a lot of production music is uploaded to YouTube. People are also able to rip music from SoundCloud, iTunes and even YouTube (for that delicious double / triple compression effect).
However, the likelihood of a company that could pay for a license actually trying to get away with this is low – most sane people wouldn’t risk their business to save a little bit of money. So we don’t need to worry too much about it. In fact, I’ve actually landed a few gigs because of fan made videos on YouTube, as have friends of mine (including one that recently got hired for a big budget film).