[sokoloff]

Correct, but the GPL demands that once you've given the source to your POS operator, that they are then free to share it with anyone and everyone.

So, in your hypothetical, it turns out to be very hard to prevent the distribution of the source, unless all the POS units are held in-house (that you are a company developing and using your own POS units in commerce, as opposed to being a POS vendor) and that all your employees who have standing to receive a copy of the source have some compelling reason to not distribute it.

That's a feature to GPL advocates and a bug to GPL detractors.