I had already answered this, but I'll try again, using your example.
LDS sells DR XT's for $145. DRiS sells them for $125.
When LDS price matches DRiS for a customer, they are taking a sale away from DRiS. If they do that enough, DRiS will go out of business. Once that happens, there are no more DR XT's for anyone at $125. That screws all the customers.
If we want to continue to be able to buy DR XT's at $125, we need to buy them from the place that offers them at $125.
In addition, if Joe walks into the LDS and buys DR XT's at $145 and then I walk in 5 minutes later and buy DR XT's, but they sell them to me for $125 (whether it's to match DRiS or if it's just because they know me and they like me), then they have just screwed Joe. Basically, they've said (by virtue of their actions), "we'll sell these DR XT's for $125. But, if you blindly trust us to give you the best deal we can, then we're going to sell them to you for $145 instead." Tomorrow, Joe and I meet at the pool for our training class and we're comparing our shiny new regs and I mention the great deal I got at $125 and now not only is Joe screwed, but he knows he got screwed. That is not good for Joe or for the LDS. How do you expect an LDS to build up a loyal following of active divers when they keep having customers end up feeling like they get screwed by the shop if they just trust the shop instead of doing a bunch of Internet research on prices before they buy anything?
I didn't really "get it" when the owner of one of my LDS's told me that he doesn't do price matching or any kind of discounting. He said that if he gave me a discount because I'm his buddy and then sells the same thing to the next customer for MSRP, that makes HIM an a-hole. I couldn't see how he would ever stay in business selling everything at full MSRP. It turns out that he is doing really, really well. One of the reasons is because his customers trust him and so they are loyal to him. One of the reasons they trust him is that they know if he says "the price is $145" that he's not going to turn around and sell the same thing to someone else for $125. They know they can get it cheaper online. They know that buying from him means a level of service that can't get from an online retailer.
The people that pay more to buy from him are doing so because they have a relationship with him that includes loyalty and trust. And that trust would be broken if he told them "the price is $145" and then he sold the same thing to someone else for $125. If that happened, they would know that when he says "the price is $XXX", he is, essentially, lying.
That specific type of lie is what those price-matching LDS's are doing that is screwing their customers.