From what I have observed, the lifeguards do not hand out fines. I doubt they have the authority to do so. While the ordinance is not posted in full at Shaws, etc. it is cited on the sign. If you think a posted rule is not enforceable, try running a stop sign with a cop sitting there and see what happens.
The guards appear to try and remind people of the rules before they enter the water. Some of the guards actually have offered divers snorkels. Some guards can be jerks, but so can some divers.
If you are a repeat offender or blow the lifeguards off, you can expect to find the police waiting for you when you get out. I suspect that what comes next, a warning, fine or free evening accomodations, is a function of diver attitude.
The ordinance dates back to the 70's so the agencies the city would have been looking to would have been NAUI, PADI, LA County, SSI and YMCA. As far as I know, none of these agencies currently train beginning recreational open water divers to dive without snorkels or buddies.
While Laguna Beach is unique in its approach to diving, it is the city's perogative to be so and the rules are not totally irrational. They are easy to comply with and also easy to get around by just avoiding the lifeguards. Dive early or late in the day during the summer, or dive during the fall, winter and spring, or just find a friend and a snorkel.