One reason a mathematician may want to patent a practical use of his idea is because if he doesn't do so, someone else can. If some corporation spent the money to understand a mathematical construction and then patented its application, not only does that corporation stand to make a lot of money (on a construction the corporation was hardly involved with), it can also keep competitors from using the ideas. Or, the corporation can "bury it," by patenting the ideas and then not using them, but still using litigation to prevent others from putting the ideas to good use. Once you patent, you control the rights to the intellectual property, and can make the product more or less widely available. To me, the patenting of an application of category theory is not an issue; the problem would be if someone patented such an application of category theory and then restricted its use or attempted to make undue amounts of money from it. [For admin and other information see: http://www.mta.ca/~cat-dist/ ]