Your argument is vacuous because you aren't translating between "revenue" and "profit." If that "15% of revenue" was also "100% of profit", then they're doing exactly what you'd expect a non-profit to do. The real questions are:

1. how much un-needed overhead their expenses consist of, beyond the minimum costs required to run the facility;

2. the dollar-amount of donations that reach the relevant causes by having this type of facility operating, vs. the dollar-amount of donations that would be given to the same causes if this facility did not exist, or if it tried to operate on slimmer margins (by e.g. only being a marine-wildlife gift shop or somesuch.)

My own suspicion is that the answer to #1 is "not much", and the answer to #2 is "a far larger amount." (Under the argument: people don't bother to donate money to wildlife preservation if they're not being actively reminded of the existence of wildlife in need of preservation. That "15% of revenue" still likely ends up as a larger absolute dollar-figure than the total amount of donations to the same causes made by all other individuals or organizations in the area.)

Even if you don't agree with my suspicions, I think those two questions are the only good ones to be debating here.