1. Because you didn't have to take a router from Plusnet in the first place you could have used your own.
2. When you took the router you had a choice - pay up front or commit to remaining a Plusnet customer for a year when they would wipe off the charge. YOU decided to break that agreement.
3. Before Plusnet could send that router to anyone else it would have to be refurbished, tested and repackaged (without that it would be worthless to Plusnet) - that would cost considerably more than £40.