Because a slip on the stick to full power just means you go faster. You don't get thrown out of a microlight. A slip in the throttle in this you are tilted at a more extreme angle, or turning a tighter turn than normal. All that is keeping you on is your thigh.
Accidents happen.
Well that is guess work as it hasn't flown, so we don't know what it's capable of and what force is exerted on the body. It's a light aircraft and so doesn't need to be safe.
I don't really understand what the safety issue is, unless you think it's going to be used instead of cars, which isn't going to happen. It's going to be a weekend toy like paragliders and Microlights.