Visitors can solve this problem by not taking their phones into the embassy with them. Although I think the real problem is that his mates have been smuggling him kit that he's been using in ways the embassy don't approve of.
I'm not sure if he'd hacked their system before, so much as one of the diplomats had given him access. But nonetheless he had supposedly used the embassy's internal comms systems in some way, which is a rather rude abuse of his hosts. And given the guy has a conviction for hacking, the embassy have a legitimate worry about his visitors bringing him unknown electronics.
As for the "held without trial" bollocks: Who is holding him? Nobody is stopping him from leaving the embassy. He can do that any time he wants to. Obviously he'll have to face arrest for breaking bail, and then the Swedes have got those 2 rape charges that haven't yet passed the 10 year statute of limitations - which I assume they'll pursue once they can.
But that's how things are supposed to be. Open justice, openly arrived at. He's alleged to have raped two women, for which he should stand trial so he (and his victims) get to have their day in court - and those accusations can be tested.
If the US government have a go at extraditing him, much better to do that in Sweden, who have a proper extradition treaty - unlike the shit one Tony Blair lumbered us with.