They mention HDMI cables specifically. I just went into a Best Buy and asked for their cheapest HDMI cable. The salesman showed me one for $15. The Amazon basics cable is $5.49. If you've got Prime and you factor in the shipping costs of using another website, it's hard to beat Amazon's price.
Best Buy preys on the 'I need this cable in 30 minutes for the big presentation' customer, and prices accordingly. Try buying coaxial cable - it's obscene!
The main reason I buy cables from Amazon instead of Monoprice is Prime. Monoprice's shipping is always slow for me. (Last gig I was at, we bought a truckload of their monitors, though, on my recommendation. Love those guys, just wish they could ship faster without ruining my wallet.)
HDMI cables are IMHO the worst example. These are add-on to core purchases and when they break or are required you just want to get one. Physical stores like BestBuy know this and price accordingly. This just makes me dislike these stores even more!