I'm far from an expert on that whole debate, but my casual understanding was that they offered the same code under GPL or proprietary licenses. And, that doing so had been found legal and had even gotten (somewhat grudging) approval from Stallman and the FSF as a way to ensure monetization and adoption of GPL-covered code. His justification has a weirdly deontological logic that I don't particularly accept ("you're not making proprietary code, just causing it to be made" is pretty thin), but I think I'm fine with the result.
More generally, though, I completely agree. Misrepresenting FOSS status to dual-license isn't ok, and has all the same problems as the Commons Clause. I think there's a strong case for restricting proprietary use via a single new, non-FOSS license (probably derived from an existing FOSS license) rather than via dual-licensing or worse, added clauses.
For quite a while, MySQL AB purported on the website and elsewhere that commercial use of MySQL required a paid proprietary license, and that the GPL did not allow commercial use, only non-commercial use.
Eventually, they stopped doing that, but it took a while.
But, often, that involved straight-out lying about a FOSS license, though, and presenting the dual license scheme as if it were the near-equivalent of Commons Clause. (E.g., the old MySQL GPL or commercial license scheme.)