of course not. Doc is free to walk away from the contract but he will then be in breach of the legal obligation and will have to pay damages unless the Cs waive their right to collect.
Again, I think you're mistaken here about how these deals work, and what it means to "honor the contract."
The Celtics cannot write a contract forcing Doc to work for them for a specific period of time. Forbidding an employee to resign (i.e. creating a "legal obligation" against it, as you say) is indentured servitude.
Empirically we see NBA coaches resigning all the time without any mention of punitive measures.
What seems most likely here is that the contract primarily binds the behavior of the team, not the coach. Loosely it probably says "If Doc wants to keep working, the Celtics must pay him under the terms described."
This is why we routinely see teams buying out the remaining terms of coaches' contracts, but not the other way around.
So, using the term "honor the contract" here seems to me totally misguided.
The Cs might be able to include clauses saying that Doc needs to give appropriate notice, or that he cannot go and work for the Knicks for a year, or that he can't make tasteless jokes about Danny's mom - basically, that he can't do something that would damage the Celtics as a franchise.
You also seem to make this argument, implying that what Doc has done is "broadcast to the whole world and free agent class that coming back is a day to day thing" and "play out [his] wavering in public" because this "unnecessarily hurts the team."
But of course, Doc has done nothing of the kind. Doc himself has not uttered a single word on the subject, other than to say, when interviewed immediately after the lass loss of the season, "I haven't thought about it yet."
The only things we've heard since then have come straight from the mouth of his boss, Mr. Danny Ainge, who says everything is fine. It's hard to see how Doc can be perceived as doing something harmful when he actually has not done anything except go home and play golf.
And if the argument is that Doc is somehow acting in bad faith by even contemplating resigning - setting aside the fact that one would need psychic powers to settle that mystery - well, we have a direct quote from his employer saying that what he's doing is totally within the bounds of any implicit agreement they had.
I'm honestly baffled by the idea that what Doc is doing represents not "honoring" the contract, in any legal or even ethical sense.