In his first broadcast interview, to the Victoria Derbyshire programme, Babar Ahmad, 41, said he had "not knowingly" supported Osama Bin Laden.
Ahmad was jailed in the US in 2014 after fighting a record eight-year campaign against extradition.
He condemned so-called Islamic State as "alien" to his heritage and beliefs.
In 2014, the former IT support worker pleaded guilty in the US to two counts of providing material support to terrorism.
Two articles on pro-jihadist Azzam Publications website had urged Muslims to send money and equipment to the Taliban.
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Watch Victoria Derbyshire's interview with Babar Ahmad in full on 14 March from 09:15 GMT on BBC Two and the BBC News channel or online.
US judge Janet Hall said Ahmad had shown genuine remorse for setting up the websites, but she added: "You can't walk away from the fact that what you were doing was enabling Osama Bin Laden to be protected in Afghanistan."
Ahmad said he had "not knowingly" supported Bin Laden as at the time he did not really know what the al-Qaeda leader "was all about".
He explained: "But technically, yes, that's what was happening at that time, that the Taliban due to their failure to hand over Bin Laden and it wasn't known at the time - and I didn't know at the time - that 9/11 is being planned and what Bin Laden is really up to.
"I did it in good faith but, in hindsight, I regret doing that and it was naive of me to do that, because it was a complicated situation. And whatever was going on then, I didn't have to advocate support for them."
British authorities never charged Ahmad, but he admitted he was a terrorist "in the eyes of US law".
He said: "At the time I pleaded guilty, I'd been in prison for nine-and-a-half years without trial. I'd been in solitary confinement for just over a year. The prosecutors offered me a deal. They said, 'Hey, plead guilty and you'll be back in England within a year.' So I said, 'Where do I sign?'
"Any person in their right mind would just sign the dotted line, that's exactly what I did. I don't regret pleading guilty, I want to make it clear. I'm not taking that back - it was the best decision of my life, and I'm proud I made that decision."
But he insisted he would have acted differently had he been tried in the UK, which is what he had campaigned for. "I would have gone to trial because, at most, I was facing a sentence of about two years, of which I would do half if I were found guilty."
Ahmad's July 2014 sentence of just over 12 years took into account the 10 years he had spent in jail in the UK prior to extradition - so he was freed in June 2015.
On the rise of the so-called Islamic State group, Ahmad said: "From what I've heard, getting people, getting journalists and cutting their heads off on TV, I don't recognise this. This is alien to me. Jihad is something to be an Islamic history and heritage and beliefs. It's a noble act that's meant to protect and defend innocent people from acts of terror.
"And when terror and misery is brought to people under the label of jihad, it's not jihad, it's God knows what it is but no god tolerates terror and misery being brought to innocent people on behalf of some sort of cause."
Ahmad said he could understand why some people felt outraged about what was happening to Muslims in some parts of the world - but he urged young men attracted to militant groups to stop and think.
"Ultimately, we are responsible for our actions and it's important that before people decide to take a course of action, before you translate your outrage into action, think deeply and carefully about what you are doing. And don't allow yourselves to be a pawn," he said.
"Don't allow yourselves to be used by other people. Don't let anyone bully you, that the only way to paradise is by bringing misery upon innocent people who have done nothing to you. Make your own mind up. Be smart. Be intelligent. Do your research."
The Victoria Derbyshire programme is broadcast on weekdays between 09:15 and 11:00 on BBC Two and the BBC News channel.
A British man jailed over a website considered a key moment in the birth of online jihadist propaganda said he had been "naive" to support the Taliban.