Canonical do not own "Ubuntu" (that is, a combination of lots of other people's software). They do not require copyright assignment for every component of Ubuntu (because they don't own it).
They require copyright assignment for a few of their own projects... which, yes, is entirely their prerogative.
BUT:
There are plenty of people in the FLOSS community who don't believe that's an optimal way to participate... and I'd point out that those people include FSF ("software freedom") folks, people who once also had copyright assignment policies, projects that have suffered from copyright assignment policies, etc.
They require copyright assignment for a few of their own projects... which, yes, is entirely their prerogative.
BUT:
There are plenty of people in the FLOSS community who don't believe that's an optimal way to participate... and I'd point out that those people include FSF ("software freedom") folks, people who once also had copyright assignment policies, projects that have suffered from copyright assignment policies, etc.
It's not just an FSF my-way-or-the-highway issue.