The intelligence community works on the principle that it's valuable to have information that other people do not. If they outsource the spying, then other people have that information, right?
Australia, UK, New Zealand, Canada, and the US are all part of the 5-eyes, which have a broad agreement on sharing intelligence.
http://en.wikipedia.org/wiki/Five_Eyes
While they may not share information with say - China - they will share it with one of the other eyes, who could also lend a helping hand in domestic spying.
but if the people you outsource to you are their allies, they may be able to justify it. Besides, it seems the NSA has a gift for rationalizing things away
Current revelations aside, when agencies are prohibited by law from engaging in certain kinds of surveillance, looping other countries in can be a way of purifying the information they desire. In other words, it's not illegal for the UK to spy on US citizens (and vice versa), so if the US maintains good relations they can get backchannel data all they like.
While they may not share information with say - China - they will share it with one of the other eyes, who could also lend a helping hand in domestic spying.