I think it's a reasonable question. It would be great if one of our online attorneys would weigh in on these issues. I am certainly not one of them, so take my thoughts for what they're worth!
Like most liability issues we face, it all has to do with "standards of care." If someone gets injured, and it could be demonstrated that the injury was somehow related to the tune of the skis, and that those skis were tuned by a shop to a condition below some reasonable standard of care, there could be legitimate grounds for a law suit.
Suppose someone skids off the slope into a tree because freshly tuned skis were dull as hoes. Or someone catches an edge and hits a lift tower--or another skier--on skis that were badly railed or concave. Either of these accidents could possibly be directly linked to the poor tune of the skis.
I'm not aware of any LEGAL standards of care regarding ski tuning. In other words, there are no laws that I know of regulating how skis should be tuned. But there ARE, I would argue, INDUSTRY standards of care. And if a shop advertises anything particular about their tunes--"razor sharp" or "perfectly flat," or something, they may fail by their OWN standard of care.
Again, I'm NOT an attorney. But I do suspect that, if a professional tune fails to meet some legitimate standard of care, and that poor tune could be linked to an injury, whoever tuned the skis could be on shaky ground.