Same reason why the top ad on Google when you search for a brand is for the brand itself -- "brand bidding" is a thing, and if you don't bid on ads for your own brand, a competitor will. If Google can do it why not Amazon!

I do not understand why sellers accept this practice. I'd assume it would run afoul of some trademark law but the distinct lack of lawsuits indicates that either the legal grounds are shaky... or the sellers themselves find some advantage in it.