They could arrest you to interview, but its agreed they won't.
If you decide to not bother doing the voluntary interview, arrest becomes necessary, and you get arrested.
Arrests can show up on enhanced DBS checks, and are a real pain in the hole to explain to employers down the line. Hence there being a non arrest option.
So you're fine that they "could arrest him, but won't" for a retweet of a stupid meme?
Who cares by what laws or twisted logic they justify it.
I disagree with that law itself for a number of reasons (its fucking daft), but the correct procedure was followed here.
If interviewing is required then what is to stop somebody from reporting somebody everyday and requiring the police to go out consistently?
I'm none of the above (thank god).
What doesn't make sense is calling that "voluntary", because it clearly isn't voluntary when the other option is be arrested.
Its a somewhat confusing term, but that's what they call it.
Usually its made clear to you at the time which of those cases its going to be.
"Look, you can come in for an interview at a time that suits both of us, or we will have to go and get you at a time that suits us. Either way, you will be interviewed."
A "voluntary interview" may or may not come with the alternative of arrest.
In this particular case, it did.
In some cases, it doesn't.
A voluntary interview may also lead to arrest (eg: if you admit to a crime, they usually arrest you to charge you).
The police hand it over, is why.
Not just anyone can request a DBS check, but many jobs require one.