Looking at someone twice, that's fine. Talking to him, maybe arresting him for loitering if he doesn't move, hey, the law allows for that as well.
Sneaking up behind him to tackle him, that seems excessive.
Questiton: "But Chairboy, what if he was a suicide bomber with bombs and suicidal tendencies and, oh dear, my glasses have fogged up with tension! I get such terrible gas when I'm tense! What if he was a bomber? He was just standing there! He could have been anything! Oh, my forehead, I'm sweating just thinking about it! I need to use my handkerchief, oh dear..."
Answer: A fine question, mosgood! If we are to ask why we can't assume he's a suicide bomber, the corollary must also be asked of 'why shouldn't we apply this standard to every person who loiters?' I expect the answer should be pretty self evident! Any time any person is ticketed for loitering, that means that they disobeyed the instructions of an officer that told them to 'move along now'. Perhaps we need to standardize the enforcement of the law in this post 9/11 world to make sure that all of these people are properly tackled, arrested, and that their belongings are 'disrupted' by water cannons. After all, in this post 9/11 world, we can't assume ANYTHING! ....except that there are a lot of terrorists. I guess we can assume that. And that anyone (ANYONE!) in the street could be plotting an attack THIS MINUTE!
Question: "Oh Chairboy, won't you please think of all the little children?"
Answer: Indeed, and I do. I think my children deserve to inherit a world that doesn't have all the sharp edges filed down. Liberty is pointy. That might make it uncomfortable to hold, and maybe it doesn't make a great pet, but it's a powerful, fearsome force to be reckoned with and I wouldn't trade it for anything, especially not comfort.