JHE
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He wasn't found to be "innocent", the jury simply preferred the cut of his jib.
The jury decided in his favour. They did so by majority. IIRC, one juror was ill and was excused and seven of the remaining eleven voted in Sheridan's favour and the judge accepted that majority verdict.
We do not know why those jurors decided in Tommy's favour. It could be, as I think Dexter believes, that the jurors believed what Tommy said and (so) believed that most members of the SSP exectutive were conspiring dishonestly to do down Tommy.
Alternatively, they may, as you say, just have liked the cut of Tommy's jib.
They may have disbelieved Tommy but thought (wrongly, IMO) it was better to find in his favour than to let the News of the Screws off. Perhaps they (rightly, IMO) dislike tittle-tattle in newspapers. Perhaps they liked Tommy and agreed with his anti-poll tax campaign. Perhaps they thought it didn't matter that he was calling most of the SSP leadership a bunch of lying scabs.
We don't know their reasoning - or lack of reasoning.