There is a difference between security and privacy or freedom of use. Locking down the device to only allow a subset of apps that Google has some control over (by requiring developers to register) is a measure that can increase security, even though it obviously takes some control away from the end-user.

The fact that the play store is not exactly known for exceptionally high standards w.r.t. malware, or that there are lots of valid concerns that come along with a company controlling who is allowed to supply apps for the device is a different topic.