UL is not a lawmaking body, so if they have regulations against being able to hush a smoke detector -- and I highly doubt that -- Nest is technically under no obligation to obey them.
A smoke detector company sells a safety product, if that safety product fails they could easily be sued for the lifetime earnings of a dead individual (or even family in some cases).
So that requires that these companies have insurance, and in order for insurance to cover them they must meet certain insurance dictated requirements, which can be anything, but UL specifications are an easy thing to point at (to both protect the company from liability and insurance from payouts).
So while in some locations there might be no government regs, insurance is making regulations all on their own, and these companies can either play along OR risk paying out up to millions of they get sued.
In effect, they are. Many regulations reference their standards, require devices meet a specific UL standard, or recommend that devices be stamped with a UL approval.
