Redhat's strategy is a bit more complex than that:
They only support the official Redhat release, but that contains Redhat's trademarks, which you are not free to copy/modify/etc... and indeed, you have to pay for the official release.
I think it ends up being that if you need the official Redhat, you pretty much have to buy it, and buy a long-term-ish support contract.
In any event, support is not a public good along the lines of software: it's very much rivalrous and excludable.
In other words, redhat doesnt make money from open source. But nobody ever complains when they're held up as the poster child for how to make money from open source.
They only support the official Redhat release, but that contains Redhat's trademarks, which you are not free to copy/modify/etc... and indeed, you have to pay for the official release.
I think it ends up being that if you need the official Redhat, you pretty much have to buy it, and buy a long-term-ish support contract.
In any event, support is not a public good along the lines of software: it's very much rivalrous and excludable.