Or at least encourage hacks for vB Lite - at the very least, people will be learning PHP and MySQL
Though I do understand the worry of having a free product start to outpace the paid one, I'm sure that if you have a similar clause in the vB Lite license concerning not redistributing a hacked board you'll be drastically reducing the number of possible "5cr1pt k1dd13z"...
If you want to be more Draconian, you could restrict the distribution of hacks and hack instructions as well - but that might eventually prove a bad idea as the company grows larger: people would switch to full-featured UBB/vB look-alikes that are covered under the GPL...
Oh well, I'll stop ranting now