Why do the Bitcoin RPC Calls use decimals for coin amounts?
I would say the RPC interface is intended mainly as an application interface, but it's also meant to be simple enough for a human to use.  And the computer cares less than you think.

For human users, using decimal bitcoins (rather than integer satoshis) makes life significantly easier.  For people writing software to interface with it, it makes their life only infinitesimally more complicated (you have to write maybe one extra line of code to insert and parse the decimal point).  The computer running it takes maybe a few more nanoseconds to run the code, which nobody will notice.  So on balance, it seems like more good is achieved by including the decimal point than not.