I think there is a fundamental disconnect here. Let's say that the MAP on the computer is $800, so the shop is selling it for $800. But, some online shop is selling it for $750. In both cases, the seller's cost could be $400.
The disconnect is that you think that the shop can mark it down to $750 to compete with the online shop and still make money. I think the real situation is that many shops are not really making very much money at all, even when they sell at MAP. In this case, it may seem like they would have a profit of $400 when selling an item for $800 that only cost them $400. But, that doesn't account for all the overhead that the brick-and-mortar store has. Insurance is a HUGE cost in the scuba industry. Rent. Staff. Equipment maintenance (e.g. compressors, etc.). I think a shop could easily put themselves out of business by consistently selling below MAP.
That said, one could respond with the thought "they should run their business better. They should be able to sell that item with a 10 - 20% margin and be profitable as a business. If the overhead of other aspects of the business (like fills or training) are resulting in higher overhead, then that extra overhead should be amortized over the business functions that require it, not spread over the ENTIRE business. Selling a dive computer does not require you to have a fill station, so don't build any of the costs associated with owning and running a fill station into the cost of the dive computer."
Unfortunately, I think the whole industry has evolved to a model where that notion is not really feasible. If, as one example, a shop made tank fills its own self-sustaining business-within-a-business, the cost of fills would be too high to sell fills at all, unless they have an effective local monopoly.
The online shop can sell the same item for $750 and make good money because their business model does not include paying for high-dollar retail space, they don't pay for high-dollar scuba liability insurance, and they are not subsidizing other parts of their business.
I think most shops do fills as a loss leader. I suspect many also offer training at somewhat of a loss. They HAVE to make up those losses elsewhere. Either that or price themselves out of the fills and training market. Or go broke.