Nah, it's that Best Buy doesn't make most of its money on the items they price match. Their big sellers are their mobile department and all the accessories folks like to pick up with their new purchase, which are very often store exclusives and have incredibly high margins. They make a killing on chargers, adapters, phone cases, you name it, but actually make tiny margins on a lot of their "core" items. The big ticket item they price match is often kicked back by the vendor anyway, so the store doesn't lose out when they do it. Their goal is just to get folks in the door, and let the momentum of a good sales interaction close the deal. (Questionable on the "good," they're admittedly not very good at screening / training, but in aggregate this strategy works well for them.)

Best Buy price matches because it gets people in the store. I don't believe for a second that they did it in response to losing business to online storefronts; rather, I think it's far more likely that they did it in response to other retail chains doing the same. Their customers came to expect it, so they obliged, as a marketing tactic to make sure their retail competition didn't have that as an edge. Their price match policy is also pretty strict; they won't match just anything on Amazon, it has to have some degree of verification and be from a trusted seller. (Especially for things like iPads / common fake listings, etc.) Last time I checked it had to be "Sold and shipped by Amazon" but that might have changed, it's been a while.

(Source: I worked at a Best Buy store for about 4 years.)