There are several reasons that could easily have prompted Shaker Aamer to consider a compensation claim from the UK government:
- He isn't a British citizen but a British resident -- in itself, this led to the (Labour) government taking a while to even get around to thinking to ask for his release, which sucks.
- For the last six years he has been the only British resident in Guantanamo Bay. All the rest were released in 2009 or earlier. That's a shocking six-year delay and also sucks.
- Then there's this allegation, courtesy of the Independent, that the UK government has spent a lot of money and effort on withholding evidence that could have been used to prove his innocence:
http://www.independent.co.uk/news/world/americas/letters-raise-fears-for-last-briton-in-guantanamo-6804791.html . If true, that's also pretty damning. People should have the right to defend themselves in law.
- It's alleged that he was tortured, and that British agents were present during some such sessions. Even if not actively involved in torturing Aamer themselves, if anyone in the UK Government knew what was going on and didn't make efforts to stop what is an illegal and immoral practice, then he would well have a right to claim compensation on the grounds of complicity.
- Since 2007 the US Government has admitted that it has no evidence against Shaker Aamer, and he was also cleared for release. Despite that he has spent eight further years incarcerated, without any charges brought against him. Without even a case against him.
http://www.theguardian.com/world/2013/apr/20/british-resident-guantanamo . The allegation made in that article is that the UK Government (or people on its behalf) were making only token gestures of trying to release Aamer because if he did come here he would have some damning evidence about torture and we didn't want that to come out. Bad luck for him that he may even be the victim of a shabby attempt to cover up the shadier side of Intelligence services.
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Possibly there are a few other reasons besides. Frankly the UK Government should consider themselves lucky that he has decided not to sue the crap out of them for the experience he went through based on apparently absolutely nothing other than his being in Afghanistan in 2001.
And despite all that, not only is he not suing the UK government, he's even spoken out pretty strongly against Muslim extremism.