I note a lack of information in that article - also no surprise.
Allowing hired ones but not private is, to me, deeply distasteful. There will be many who perhaps could benefit but would not always be comfortable with what rental companies offer. This could be due to "disability" issues. And others who live in places without such schemes.
Presumably this is supposedly a means they think they can monitor the experiment so if it goes very bad, they can make them (in theory) illegal in public spaces again. They wont want people buying them and then being told after the trial - hey , we changed out mind, you can't use them now.
As a non driver , a scooter might have appealed to me, as a cheaper option than E Bikes, if it goes well, I imagine they will then produce standards rules etc for individual purchase and use but there are too many idiots and kids around who will undoubtedly spoil it for everyone.
And yeah, see them around in London being used already without a care.