I can't answer for the UP. But on the BNSF and Federal Law. Quiet Zones are a nice thing, not a necessity.

Some Light Reading -- ecfr.gov

Now GCOR 5.8.4 is the rules that cover whistle quiet zones. There are a ton of exceptions where a whistle must be sounded, like emergency, automatic horn system failure, grade crossing malfunctions (6.32.2), grade crossing out of service, supplemental or alternative safety measures are not compliant, other railroad rules (such as main on the right of way).

Have I whistled before in a quiet zone, absolutely. There was an immediate danger to a trespasser at the time. And as a MOP told me once, "Lawsuits are expensive and compressed air is cheap! Blow baby, Blow!"

Now that some people are starting to get far reaching and attempting to sue the crew on board, don't like the way that the public handles it.