If the tire shop here charged extra for having slime in the tires then they wouldn't have any business.
You really don't want to get stranded out on a dirt road in the desert where towns are often separated by 50 miles or more, and most of the towns are too small to even have a tire shop.
It's pretty much a necessity in an area covered by plants that have some sort of thorns growing on them (mesquite, cactus, yuccas, cat claw, etc.).
Not only that, but you've got dead greasewood stumps, sharp rocks, and a million other things that'll flatten a tire as soon as you unload the quad from the trailer.
Even animals will flatten a tire, it's not uncommon for a tire shop to find a porcupine quill or a rattlesnake fang still embedded in the tire.
Shops around here require heavy gloves when working on tires...one guy lost his arm and another nearly died from running his hand across a fang while feeling for what caused the flat.
Last shop I went to had a pressure washer with a custom made nozzle that sprayed in a 360 arc, so they could spray the inside clean in a matter of seconds.
Besides, everyone who works at a tire shop has that same dirty look, you can tell who they are just by looking at the black crud all over them.
I've yet to see a tire mechanic who was remotely clean in appearance...and if you bring them a brand new tire with a clean wheel, it will wind up being dirtier than when you brought it to them.
I don't see how slime could possibly make them or their shop look any worse