I think the Police were right to arrest him. Essentially all they are doing is causing him grief and inconvenience, which is rightly deserved. Plus now he is on the DNA database, and should he do anything similarly stupid in the future they will have him on record.
But I highly doubt they will charge him with anything, but it sends a stark warning to any other nut jobs out there considering doing the same.
You think it is acceptable to take a persons DNA for saying something nasty?
What a slippery slope we face.