CTlawyer,
1. The owner of the hall is certainly a participant. It may well be prohibited as mechazek yadei ovrei aveira. The caterers might not be as they are only selling food. In any case, as there are others who will gladly take money to participate do there is no reason not to grant a religious exception. In any case, the law is requiring them to do something – sell or provide a service for these farces.
2. We were discussing America.
3. If it can tell him that he cannot perform a ceremony (e.g. a polygamous marriage) why can’t it tell him that he must perform one. Solicitor General Donald B. Verrilli Jr. does not preclude the possibility. For that matter, why can’t it revoke tax-exempt status of a shul as with a religious university. Why can’t it stipulate that the person’s license to perform marriages will be revoked? What about religious judges as this is not one of their regular functions? On the other hand, if accomodations are made for taking off on religious holidays why not for this (Prof. Eugene Volokh once suggested this).