Re: Apparently he was allowed to continue his journey
It's not really plod who decide on tweets tho it's the CPS, a cop can't really say "excuse me sir, but that looks likes you're about to do something against the law, you see how this is both dumb and illegal, and you agree to not do it anymore?" when someone is twattering. Might be good, although I prefer the option of informing the posters mum of what they said.
Tweeting is also considered more "public" by some laws, since it can be seen by many people. Not saying I agree, but laws have their own logic.
But in a face-to-face situation, the plod can make such a call (subject to whatever directives they have). Since this could probably be filed under "plonker with a toy gun" a warning and public service announcement should be enough. Arresting him would have been a waste or resources.