I think it's a typical knee-jerk reaction to events. Samurai swords make fantastic decorations; I have an imitation one that is used for that. I do get it out occasionally, but only for practising movements with it in a performing manner (I perform with poi and staff, fancied branching out, one day I may try learning it in a martial arts sense), and I imagine most people who own one have it for these reasons. Yes, it would be nicer to have an authentic one, but few people can afford them or want to pay thousands for something to put on their mantlepiece.
Banning them isn't going to stop crimes being commited with them, hell it's now likely that loads of people are going to go out and get one before they're banned. Yet again it's an example of how our society and government tries to solve crime problems by looking at how they are commited rather than why. Did banning guns stop shootings? No, and this ban won't stop a commited nutter from getting hold of a sword and hurting/killing someone with it.
Of course there's a clear danger posed by them, as there is with kitchen knives, hacksaws, cricket bats, hammers etc etc, all of which I might add are just as dangerous, some more so, than imitation swords, which are sold with blades so blunt you couldn't cut butter with them. Yes you could thrust one into someone, the tip is the only sharp(ish) part on them, but you could also thrust many readily and more easily available objects into people.