I'm really not seeing the distinction here. If a company offers a salary and includes on-call as part of the deal (and communicates that up front so it's not a bait and switch), how is that different than running your own business and getting compensated for your on-call time as part of a package that you sell to a client? In both cases you agree up front that you will be part of an on-call plan. In neither case are you getting a bonus for doing a good job at on-call, because either way you're just doing your job that you committed to ahead of time!
I'm totally sympathetic to people who don't want to be part of an on-call. Jobs that have on-call aren't for you, that's fine. But I don't get this idea that it's uncompensated labor, unless there are tons of people out there who somehow ended up in jobs that sprung on-call on them without warning.