Wobbly Software

Why is it that software I've been using in demo mode works fine until I pay for it? I've been using this DVD software (in an entirely legal and above board way I might add - converting my collection of home made birdsong DVDs into a portable format) which worked really nicely until I decided to bite the bullet and actually pay for the darned thing. At which point it starts rejecting specific DVDs which have really good bits of birdsong on them. I wonder how it knows that I've now paid and it no longer has to pretend to work anymore? Oh well.