This is much the same debate as has gone on over similar tactics by PRS in the UK. On the one hand, it's only right that a composer gets recompense for his work, if its played. It's almost certain that at some point in a session something will be played which is copyright. It's also unrealistic to expect (a) session musicians to put in a detailed return of what's been played and (b) to expect PRS/IMRO or whoever to analyse it in detail and charge the venue accordingly. So charging a fee in the expectation that copyright material will be performed has some logic.

On the other hand, when it's non-commercial performance then charging the venue an unrealistic fee seems over the top. Especially when the mechanism for distributing the fees to composers is so crude that anyone whose music gets played in a folk session is unlikely to get anything.

I've mentioned it before, but a folk club I was involved in was nearly shut down because the PRS jobsworth based the fee on the potential capacity of the room rather than the size of the average audience (and also because his attitude offended the landlord),