I'm confused....

I've watched this a couple of times and it just occurred to me...
Why, if S.A. Butan is working for them, would he get the police to work against them.
The cops were partners already and could have done the same thing as part of the con, just without the aggression.

Help me understand. It just seems like an unnecessary inconvenience.

SPOILER ALERT!!! Good guy wins/gets girl, a-hole gets kumuppins & I'm out $8 SPOILER ALERT!!!