In European copyright law, they actually can't: non-exclusive (eg. can't be sold) moral right protects the obvious moral aspects, but there are also compensation related clauses (eg. I created a "work of authorship" and got compensated $1000, yet company earned $5 million off it, I'd be entitled to ask for a fairer share of the profits). I am sure this hasn't been tested in courts for software development so far, with it being such a collaborative endeavor, it'd be hard to lay a claim to how much your worm was really worth).

I also take a different view: a person is not owned, but it's normal for labour to be paid. A labourer can design their own pricing scale to encourage labour deals they prefer. Just like companies design their pricing schemes to eg. support only large customers (up to 20 users free with no support, paid-for afterwards).