In the shops I spend my time and money in, if you have to purchase your supplies for the class from them, they tell you in the class description, otherwise, the are just glad you have come to learn and enjoy. The one shop that does not "make" you buy their yarn but treats you awful ... I don't go there... because they aren't very nice to anyone!
If I had an LYS, or any business for that matter ... I would want people to come in no matter what ... and who can leave a LYS without purchasing something... or ... thinking about something so much that they go back and get it