I think the salient point is not just that suspicion isn't court-tested proof, it's not necessarily anything. In any police investigation, people may at some point be suspected - but perhaps only to be ruled out later, or perhaps on the basis of information that's entirely wrong. The fact that somebody gets nicked doesn't just not mean they're guilty, it can quite often mean there didn't turn out to be anything on them - which itself doesn't mean the cops were wrong to nick them.
But you can't be releasing people without charge and then deporting them because intelligence-officer-taps-nose. Not in a democracy.