No, it's not, because he's clearly being investigated for supporting Hamas' actions (which classified as a terrorist organization by the UK).
If the government seriously thinks he's capable of following through on violent acts because he wrote a Tweet saying "I support Hamas", I suspect the government is badly mismanaging its anti-terrorism resources. If the government doesn't think he's going to follow up that Tweet with action and is "merely doing it because the law says they can", that's pretty much the definition of harassment.
Whether or not that’s enough to search him is another debate, but that’s what their thought process was when they decided to detain him.
If they think that then why just him and not the thousands of other people tweeting similar things?
The thought process is evidently "oh, it's that bloody git again, have we got anything we can pin him for this time?"; if you don't see that it's you who's being naive. (I'm agnostic about whether it's specifically about Scottish independence, the Assange case, or something else; one way or another he's a thorn in the side of the powerful, and that's why the book is being thrown at him)