Their fees are a % of the ETF's assets under management each year, so they make money either way, but BTC price going up makes them more. E.g. $1bn assets, 0.2% p.a. fee = $2m p.a. . Suppose there's then noone buying or selling, but BTC price doubles, then assets would increase to $2bn and ETF issuer would get $4m p.a.