Chemicals in Urinal Odor Eaters
What chemicals are used in so-called urinal "odor eaters"? 


There are 1,4-dichlorobenzene-based and non-PDCB products. In the latter, the chemical mixture from the linked MSDS is:
SODIUM SULPHATE, SODIUM DODECYL SULPHATE, COCAMIDE MEA, DIPENTENE, CITRAL, LINALOOL,BENZYL ALCOHOL


  Do they actually destroy odors, or do they just mask them?


They mask odors.


  Also, are these chemicals subject to any environmental laws or regulations?


Yes, as detailed in the MSDS sheets linked above. Please note that my knowledge of urinal cakes and related products is not exhaustive.