Firstly, they are gliding CLUBS not gliding SCHOOLS. A very different beast.
Secondly most of them offer some form of bookable flying, many offer courses. At my club there is a 5-day per week instructor we can book with, plus through the summer a second instructor runs course Mon-Fri. That's not untypical by any means.
Lastly, maybe they are short of tug pilots? Except at one or two clubs in the UK they are again unpaid.