He likely would have beaten the terrorism charges, but for the lesser charges it wasn't clear that he'd get a better prison sentence than the one he was being offered.
He took the deal and pleaded guilty.
Also, lawyers cost money - perhaps he couldn't afford to keep fighting through a trial. I also have another friend who had the same charges leveled against him (not for donations, but volunteering services). He did fight the charges and got acquitted. I don't know if it's true, but I heard it cost him well over $1M for his legal defense.
[1] Trials like these take a long time to prepare for, by both parties.
Having values is expensive
Many people pay with their lives
But sure, if anyone wants to pay in this fashion, I won't stop them.