They're a company so their goal is (among other things) to make money which they do (among other things) by charging for premium membership. Assuming there's a business case for this (very cool) tool, the question is: will they make more money by allowing people to try the tool before signing up?

Seeing how the tool requires certain amounts 'points' for some of the brushes to be available, this would make the benefit of a premium account even more clear after toying with the tool.

So, to sum it up: for me, not being able to try out the tool immediately is a small frustration. Presumably, for other people without Deviant accounts this is also annoying. I believe, as argued above that they might be able to make more money if they let people play with the tool first. Regardless, that might not be their goal at all.