Definitely not something that is illegal to do and definitely something some tuners feel the need to do to ensure their customer isn't going to alter the tune because they feel they read a couple of internet web forums and are now officially tuners.
I do disagree though with completely locking yourself out of the tune. The end user should have the ability to flash back and forth between the stock file and the modified file. You
shouldn't be able to edit the tuners final tune without a good explanation imho.
I can see the argument go both ways. Regardless though, we don't lock our customers out of their tunes either. We do however charge double if you screw something up after "we told you so"
jk
Nick