for example,
britney spears is under jive, and christina aguilera is under RCA,
somehow they are both under sony music, ..
isn't backlash for each other actually a bad thing to sony music as a whole?..

another example,
rihanna is under def jam, lady gaga is under konlive/interscope and was dropped from def jam,
and they are both under universal music.. is a future collaboration beneficial for anyone?..

Even though some record companies are owned by the same parent company, they are not necessarily connected at all. They are still very much in competition with one another.

Competition = more sales = merits their respective record labels a good disposition in their parent company

It's all business.

ask 50 cent and kanye west