Roger Nichols was whining in one of the audio advertisement rags about this very thing.
Let's put some things in perspective.
Edits: Okay, so you think maybe your edit's shouldn't be released to the client? Well, in the old day of cutting/splicing tape, you would release the end resulting tape to the client correct? Of course you would, they PAID YOU to do those edits on THEIR TAPES!
Console settings: Often, the engineer would either have sheets of paper with a layout of the console on it, or would do an audio tape while he talked his way through every setting on the console. This audio tape or sheet(s) of paper would be given to the client, along with their tapes at the end of the session. All this talking and writing was time THE CLIENT PAID YOU FOR.
Plugins: So, what stopped the client in the past of writing down what was inserted on what on the console and writing those settings down? For that matter, some clients would do a SysEx dump of the effect processors after sessions. All the time you spent setting that stuff was time THEY PAID YOU FOR.
I guess we should go back to what kind of agreement you had with the client! If you have a CONTRACT that states you "produced" this session, and are entitled to future monies as a result of your work, then by gawd, you should certainly get that money. But, unless you have some kind of exclusive deal with the production, the client certainly is entitled to go elsewhere with THEIR PROTOOLS SESSIONS and have somebody else mix it, or so whatever to it.
Years ago I mixed a session on a DDA console with Uptown Automation on it. All the mix versions were backed up to disk and the client STILL has that disk. If they were so inclined to go back to that studio and pull up that mix, all the more power to them. It isn't MY production anymore. I got paid in full, under the agreement I made with them in the beginning. They paid me for all hours I billed them for, and for all media they agreed to pay for. It is all theirs! Would I be a bit miffed that they took my work elsewhere and "finished" it? You bet, but would can I do about that? It is their choice to do so, and as long as they have paid me what they agreed to, after that, they can do whatever the hell the want.
You are going to have a VERY hard time trying to find some kind of wording in a contract that would stop a client from taking your ProTools session to somebody else and having them "tweak" it a bit then take all credit. If you didn't agree to some "credits" ahead of time, well, you weren't thinking ahead!
This sounds harsh, but let's make one thing clear my man, it is about the artist, and NOT about the freakin' producer/engineer. If you are ethical and forthright in all your endeavors, you will mostly be given that same kind of respect back!