Presumably, this violates MSFT's investment agreement with OpenAI. Any reasonably competent counsel would add "no-poach" protection for a strategic investor investing in a startup, and this is as clear a case of poaching as there is.
We are though taking about OpenAI, by that time probably one of the five most valuable startups in the world.
The point is to discourage market distortion. Some jurisdictions also make employee contract conditions of a similar nature illegal too, as they interfere with personal freedoms.
A lot of business deals though specifically include clauses to prevent one partner from poaching the other's staff, as otherwise one side could do what appears to be happening here: Unilaterally taking over the entire business.
Also, in most jurisdictions, no poaching agreements are unenforcable if the employees themselves initiate the transfer. Signing that letter about leaving if Sam is not coming back probably qualifies as a resignation notice.
On top of this, most other AI leaders have already stated that they want OpenAI employees to come to them.
I have all the sympathy with OpenAI employees signing the letter and looking for new jobs.
However, MSFT is basically doing a cheap de-facto acquisition of OpenAI, a company they have invested in. This sets a terrible precedent for companies taking investment from strategic investors where they may stage an opportunistic de-facto acquisition at the sign of any trouble.
Given the relationship between the two, and the context of the matter, I don't expect no-poach agreements to hold much weight here.
No matter how smart or dumb that move was.