Reasoning I find confusing. A pardon is by definition something given to people convicted of breaking the law. It can be and is sometimes used as a relief for people who have been -wrongly- convicted if there is no other mechanism (or just because it's easier), but that's not implied or required.
Is this some difference between US and UK law?
By modern standards it's obviously a stupid, unjust and immoral law that he was convicted under; but if he was correctly found guilty under the laws of the time, then it was a sound conviction.
Which is why I find that reasoning confusing - when you pardon someone, you just do it... and if you care to give a reason you can.
Perhaps it's different in the UK? Is this some limitation on the powers of the House of Lords?
Pardons are extraordinarily rare in the UK and normally only granted for misapplication of the law
'It is the standard policy of the Government to only grant pardons to those who are considered "morally" innocent of the offence, as opposed to those who may have been wrongly convicted by a misapplication of the law.'
It also mentions that in 1996 there was a pardon given as a "reward for information".