I can definitly see both sides to this, even if it is not legal for a workshop to lock a tune, i can see why they would want to.
I believe that once you pay for a tune, it is yours. The workshop doesn't own the tune and you are "renting it", ie; using it and paying for it but don't actually own it. It is your tune.
Fair enough the workshop may have people that have been in the industry for years and know how to get the best out of a tune but for example, If i pay $x for head work, can the workshop lock the head of the engine on so no one can take it off and look at it? No. The same goes for tunes.. You pay for what you get, better tuners are more expensive, you are paying for their knowledge, not by them locking the tune off.
Also i don't believe tunes are very transferrable. All engines are different and a tune on one engine will not react the same as the same tune on another. There are too many variables to a tune to make it worthwhile transferring it to people you know.
It would be interesting to see where the liability is if the engine pops. Maybe if the workshop kept an exact copy of each tune on file, liability of tunes that have been fiddled with could be avoided.
My Opinion