It shouldn't be common practice, it's against Etsy's terms. Feel free to reply to the seller and let them know that doing that is violating Etsy's terms, as they may well not know. Perhaps they once favourited something, received a message and thought that it was a good idea to do the same thing, but for the most part, it's annoying and not well-received. Though I've not had it happen to me yet.
And I'd also say, if you do end up buying the item, DON'T use the code they gave you, as it may well only encourage them.