outland144k
Seriously, though: users endorse products they (hopefully) believe in and use, right? Manufacturers simply have groups of artists that endorse their products; they don't endorse the artists, per se.
You are technically and linguistically correct, but remember we are dealing with musicians here, not necessarily the most grammatical folks. "Endorse," "endorsement," and the very popular "endorsement deal" are expressions that have come to mean whatever the speaker wants them to mean, and it's been going on far too long for you to be complaining that people are using the term wrong.
What I want it to mean is that Gibson gives me a bunch of their ridiculously overpriced fancy guitars and sends me on a tour of medium sized venues in exotic locales around the world, of course paying for transport, lodging, sidemen, etc.
They can endorse me, or I'll endorse them, whatever they want.