I don't think that is so much the dive certification agencies trying to just make more money(even though that may be some of it) as it is a liability issue. If the agencies certified kids under the age of 14 to go over depths of 60 feet and something happened, then trust me the legal system in the US would find a way to hold the certifying agency accountable. I have dove (or 'dived' for you UK guys out there) with a group that had several kids (not mine) under the age of 14 and while they are competent safe divers in MANY ways, there are areas of skills that I saw lacking (dive planning, air monitoring, surfacing several times during the dive to talk to their sisters who are snorkeling above them). These are things that can mean the difference between a good day diving and a tragic day diving even on a shallow dive, but the risk increases the deeper the dive. Of course not ALL kids have these issues, each kid is different, but the agency has to assume that ALL kids are like this, thus the depth restictions.
In answer to your question about the 2 tank dive. I can only tell you that when we go to Key Largo, and the first dive is the Spiegel Grove my 11 yr old son stays his happy little butt on the boat until the second shallow dive. If the charter is doing 2 deep dives on that trip we will opt out and do something else and catch the afternoon dives. Generally most charters will have at least one trip a day that will do only shallow reefs.
Does it stink that he can't do the deeper dives with us until he is older, or that we sometime don't go diving because of the depths of the planned trip... ABSOLUTELY.... but I'm ok with that. I would rather he have the maturity and experience prior to going on deeper dives.