Well, rats in the wild have a 95% mortality rate before the age of 1 year. Their breeding is almost completely controlled by the population density in the area. (This comes around in the form of different types of breeding practices by males)

I have been unable to find any evidence to support your theory that they are pregnant continuously, and I haven't found any to say that they are not, except for what is stated above.

My point being, if the female rats are visibly suffering from being continously bred, then it can be called selfish. As of right now, I don't have any proof for either stance.