The point is that the free beats are gonna be used by people that either aren't serious rappers or aren't making any money off of music so by having them for free download they generate a lot more interest, go a lot higher up the charts and will probably get a few people that are willing to pay hear their beats.
Having the beat up for free download AND exclusive sale though is dumb. You should have beats that are pay only and the odd completely free one to generate interest.
I wouldn't worry about it though. If you like the beat then buy it exclusively as long as it doesn't cost a ridiculous amount considering the situation. You're probably never gonna hear anyone else not connected to you rapping over the same beat. And most likely your fans (if you have any) won't either.
If you were 50 Cent however and you wanted to use the beat on a lead single then it's a problem.