Here's my take on Brushes. Brushes are tools. The patent for the hammer does not - indeed cannot - restrict what you can create with the hammer.

My brushes themselves are released under a creative commons license. That means if you want to make new brushes out of them (say, you want to convert them to work in a different program) that use is covered by the creative commons license.

If you want to USE the brushes, I don't believe I have either a moral or legal right to restrict that use. That which is made using them is the intellectual property of the creator of the work, just like the person who makes paint brushes in meatspace can't say that you have to tell everyone that you used their brand.

I would love to encourage everyone to release things into the creative commons, but I cannot make that a condition for using the brushes. I really would love to see what people do with the brushes I create, but again, I cannot make that into a condition of use.