In a sense, they already do.
Marty Roesch – the guy who started the whole thing, has a commercial version – and a company called sourcefire.
Sourcefire and Snort are inextricably linked. Marty has also said that he’s committed to keep Snort open and free.
The bottom line – from my perspective anyway – is that Snort is great for smaller installations, but it doesn’t scale all that well. In fact, there’s a current thread on the snort users group on that very subject.
Sourcefire (the commercial product) is designed to scale better.
Hope that helps!