I've seen several cases where free or even paid software is pirated, rebranded and then re-sold, giving the original author no credit at all. Having the source code or not seems to make little difference in these cases, but it's certainly very simple to re-release a product using source code. Not everyone in the software business is full of ethics, morals or even a shred of respect for the work of others. It's a valid fear of being ripped off, even if you don't receive money for the software - not all value delivered to authors is monetary, as the vast bulk of people who write books will tell you.
The other thing with releasing open-source versions of software is that sometime, underskilled consultants build versions of your software with code hacks in it, and pass it off to unsuspecting clients. The clients then contact you for support, and you chase your tail trying to work out what is wrong, only to work out eventually that it's a rogue build of the software. This is somewhat ameliorated with a true open source project, but is always a risk if you are supporting versions of your software.
The other thing with releasing open-source versions of software is that sometime, underskilled consultants build versions of your software with code hacks in it, and pass it off to unsuspecting clients. The clients then contact you for support, and you chase your tail trying to work out what is wrong, only to work out eventually that it's a rogue build of the software. This is somewhat ameliorated with a true open source project, but is always a risk if you are supporting versions of your software.