The assumption is that legitimate companies who sell software will sign it and that signature proves it came from that company who you trust because of their publicly known legitimacy. It's a bit of circular reasoning. But it does round back towards that legitimacy - if it is found that they violate your trust, they lose public trust and thus lose sales.
Luckily new OSes (cough NOT WINDOWS) are able to sandbox applications and prevent them from accessing resources without declaring the need to access it.
And as for the signing certificate, I think the MS Store will allow any signed app. They just offer the cheaper signing service.