I recently worked on a paper with some colleagues as second author, and suggested that we publish in an open-access journal with a decent impact factor. Our university also has an agreement with this journal so we would have been able to publish there for free. However, I was turned down in favour of another journal that my colleague suggested, which had a similar impact factor but wasn't free. The reason given was that 'we have some funds left over, so we may as well use them'. As I'm still a student I didn't have much say in the matter, but it seemed quite wasteful to me.
What are the reasons for academics choosing to 'waste' grant money (if this is indeed wasteful)? Is it to save their own university money, or to artificially inflate their expenses so they can apply for more money the next time they apply for grants? Or something else?