The FBI had been after Sami al-Arian for years. I credit Bill for jumping on the story and bringing American's up to speed....but O'Rielly didn't uncover al-Arian through investigative reporting. Books have been written mentioning him as a "bad guy".
Read "Terrorist Hunter" by Rita Katz....remarkable story of an Iraqi Jewish womans fight against terrorism once she got to America!
I’m sorry for giving him too much credit.
There were quite a few al-Arian types the FBI was aware of for years, but if you read Steve Emerson's work, you'll know agents couldn't even keep open-source files, such as newspaper clippings, on these people, due to the famous "wall" and threats of lawsuits for profiling (remember the Gore/Bush 2000 debate? -- it wasn't just about African-Americans, it was also about Muslims, who (CAIR) had been lobbying for anti-profiling legislation for years).
As far as why these people aren't always charged to the max, it's a matter of building the best case the FBI can, on whatever charge. Hearsay doesn't cut it in a court of law, and if you follow some of these terrorist types, you'll find a case is being built, on a completely different charge, in the background, while their lawyers (often bought and paid for by the Saudis, through CAIR) are fending off another charge.
You do the best you can with what you've got.