You have to add the license key that comes with the license certificate (your proof of ownership) into your <book> tag as parameter like
<book licensekey="1234567890" plugins=".. .. " .. >
The key can be checked with a ctrl-rightclick into the book (when it has focus, so probably you have to leftclick into the book first).
Then you see a little about box with the menu item "validate key" and the version number.
This enables us (and others) to check the validity of the license. We indeed already got hints from users pointing to commercial users without commercial license.
We actually do not waste our time in actively searching for "abusers", but if people tend to play unfair, we might outsource that ;-)
There's also a way to search with google for license keys; an interesting feature also ;-)
Our experience so far is that our users/customers like our open and "based on trust" approach; and that companies for that little money do not want to run any risks.
Hopefully this answers your questions!
BTW, have you seen our latest tutorial:
http://www.vservu.de/mz3-tut/index-tut-t1e.html ?
REgards
Hans