I was wondering the same thing. My thoughts are that anything liquid shouldn't be allowed to freeze, at minimal you'd have a split container and a mess, and maybe the chemical's effectiveness would be compromised by freezing solid. It does get cold enough here to freeze things solid, even in a non heated garage.
I wouldn't worry as much about chemicals in powder or chunk (Brilliance sanitizer) freezing, since they're already solid.
But that's just me. I could be wrong, there's a first time for everything after all!
Brewman