I don't think the Borg allowed Seven's breast to stay, the producers did. Her physical appearance was to get folks to watch, not a representation of actual sexual physical maturity.
But it's a bit of an assumption that the Borg just cut off whatever they didn't use- it's not like they chopped off everyone's legs to give them wheels, or whatever. Presumably the point of assimilation is to get the benefits of whatever physical characteristics the target species has to them. So it may have just been a case of once Seven's secondary sexual characteristics started to develop, the Borg just ignored them, the same way they ignore things they don't consider a threat.