Sure, if you also consider people making money by selling "premium" themes to be ripping off everybody who's ever contributed code to WordPress.

People put in a lot of their time to contribute code to a open source project like WordPress. They do it mostly for free, or because they enjoy it. Then they give that code to the world, to do with as it will. All they ask is for people who use that code to do the same, and give their code to the world as well. And "give" doesn't necessarily mean "not pay" here.

So, if theme designers are not willing to do the same as the people who's work they're making a buck off of, then my general opinion is somewhere in the realm of "too bad, they can use some other piece of software if they don't like it".

That said, many theme designers are seeing the light and seeing that their business model is not really selling of the code or even of the theme. Selling a piece of software only really works as a money maker when you sell it an extreme number of times. But sell support and sell feature development, and you'll keep putting cash in your pocket for an extended period of time, even with only a few users.

Give away the razor. Sell the blades.