They could just ask people to buy a license... It doesn't have to pirate-proof, just ask for a small payment in exchange for a license key, those who want will pay for it and those who don't want or can't afford will use a pirated key. Way back, I paid for an Opera license even though I could get a free Netscape or IE, because Opera was a much much better product IMO. I also drop some money into buskers' hats when I appreciate their performance, many other people do too. I'd pay for a solid spy-free Ubuntu as well.
Trouble is, it (the licence) isn't Canonical's to sell.
They can, however, charge for support, documentation, physical medium (the DVD set), access to their servers for downloading and a whole lot of other things I can't be bothered thinking up. But not for a licence. And if they charge too much for any of those things we can all look forward to the new free (as in beer) Tatmsa 9000 distribution (which will look a lot like Ubuntu).