Plugging from a shops' viewpoint does not make sense. Why do something for $5 when you can do it for whatever they charge for a flat nowadays?
In addition, there is the fact that a small percentage of plugs may leak. Usually you can just put a different one in and they seal OK, but a shop that would only patch tires by plugging will have some bring-backs over the course of time where a shop that only patches will hopefully not.
I would hope this is a moot topic since I think that every bike with tubeless tires should have a couple of the string plugs carried, and the riders should know how to install them.