Of course, if Sam's are being honest about their true reasons for this, it is the cost of the licences which they object to. Whatever 'dodge' we can conjure up won't apply if they say 'no music on our premises.
I believe that they will come to realise that they will make more money by paying the licenses and allowing music than watching a large part of their customer base go to the competition. They might make less money than they made previously when licenses were cheaper but it will be a lot more than they are going to make in the future.
A lot of their landlords are up in arms about this and are leaving. That increases Sam's recruitment and training costs and perpetuates the spiral of falling revenue.
I feel that change is likely to come via the economic route rather than the legal route. Who will blink first Sam's or PRS?