All we're seeing here is that Google have determined that Plus is a good signal since they assume that their knowledge of that data can be trusted and verified more than the data held by a third party.
They are probably right on that, so you can imagine that they would class it as a very strong signal, such that against another resource with many weaker signals the other resource loses and the Plus page ranks higher.
The only question is whether this is in some way a bad thing.
Is it bad to place a strong signal on one of your own properties?
To not place a signal would be equivalent to negatively placing a signal as they most likely do have signals against other identity services (LinkedIn and Facebook), but to place an equal signal is to say that you only trust your own data as much as a third party - which is probably not the case, you trust your own more.
To place a strong signal is probably correct in terms of your trust of the data, but does raise the spectre of being questioned about whether this is fair behaviour.