I always assumed that it was because the favorites are the person's, but if it's on their shop account (and they only have 1 account), then it's associated with their shop.
In the brick and mortar world, usually businesses do not openly promote other businesses like that unless there is some sort of selling agreement.
And if it's a super popular/well-known shop, it could be considered some kind of endorsement, when really they just wanted to say "ohh, I like that earring. I have a wedding to go to in a month and I need something to match my dress."
It's personal, not business.