Right, I'm aware that it can't be forced on them. But from the article:
"One large supermarket chain and other high street retailers, as well as a budget airline, have asked to be included in the Bristol pound scheme but were turned down, he added."
If they voluntarily want to join, and they voluntarily run up large Bristol Pound balances, then they're going to have to voluntarily figure out what to do with that money in order to keep it in the local economy, since it "can't" be converted back into normal pounds.
The idea isn't that you're going to hold them at gunpoint and say "INTEGRATE WITH THE LOCAL ECONOMY OR ELSE!" it's that they're going to voluntarily accept the money (by being a part of the program) and then they're going to have to figure out how to recirculate those dollars in Bristol.
I'm not saying that I necessarily think this is a genius idea by any means, but if they're TRYING to be a part and being told "no", that seems contrary to the goal of "keeping money circulating in Bristol" and therefore I don't understand it.