Because the only real reason people have them is for use on other people. There is zero legal reason for anyone other than a serving police officer with his issued one, to have a stun gun in the UK, they've IIRC never been sold legally over here, they have zero uses against anything other than humans. This isn't something you buy by mistake and don't realise it's on dodgy legal ground. The likes of Crossbows have legitimate uses, but you step outside of them and you'll find yourself facing the police pretty fast (and probably armed police at that), they're also pretty obvious and thus unlikely to be used for something like, say mugging someone. Then there is the fact it's disguised to look like a torch, so even the self defence argument loses any minor shreds of plausibility it might ever have had (not that that would have made a difference, from memory it's a binary offence), and almost certainly is dealt with as an aggrevating factor, as the only reason to have a disguised weapon is to try and hide the fact you've got it - which suggests you know it's illegal. The fact it was found during a police search of his property should really tell you something about the guy, as the police don't generally do searches of people's accommodation without some fairly good reason, either because they've arrested the person for a crime that allows it, or because they've presented evidence to a judge or magistrate who has deemed it sufficient reason. Finally a google suggests he was a drug dealer, so yeah that's not going to have gone down well with the prosecution, jury, or Judge when they consider the fact he had a disguised prohibited weapon.