[sourcery]

Yes, they can define income however they like, but once defined, that definition must be applied uniformly—as must the tax rate. That's what's required for the tax to satisfy the constraint not only of uniformity, but of self-consistency and non-violation of the Law Of Non-Contradiction (violation of the latter results in being able to prove that true = false, and hence renders logic and truth meaningless.)

[Hostage]

And they do apply the tax rate uniformly with each tax bracket. They have the power to set up brackets and be uniform within those brackets.

That’s how they get around what you might consider the origial intention of uniformity.

By setting up brackets however they wish, they get a progressive tax code.