Naive world view. They also filed other less serious charges against him (of which he may have been guilty). He intended to plead "Not guilty", but after 2.5 years of him being held before trial[1], they offered him a deal. Plead guilty and get 5 years, including time served.

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.

Do you really not care to know what the terrorist organization was before you assume that "they are fighting against oppressive bullshit"? Is A/I and ISIS the same to you?