When I looked at using a CMS for a personal website, I discounted any which used GPL3, because I couldn't be sure that if I changed a template I would need to make it available, not to mention doing a small code change. It's not that I mind sharing, it's just that it's a hassle and it just worries me that if I use a GPL3 CMS I'm opening up a pandora's box. I don't know if that's really the case, but because these licenses are complex to decipher and GPL has a reputation, it's not something I want to risk. So even though it's not for profit, and I don't have a real problem sharing the changes I did, I'd rather stay away and not be liable for anything.
The problem with many of the open source licenses is that they are complex. A commercial license to source code or libraries typically gives you simple terms: certain people can use the code for certain purposes for a certain number of projects for a certain amount of money. Some open source licenses, GPL in particular, contain tons of legalese and conditions which IMO makes such licenses best avoided by most developers, regardless of whether they're used for commercial purposes or not.