All of bitminter's hashrate goes towards merged mining. Slush is only doing merged mining on his getwork nodes, which are currently only 10% of his pool (merged mining does not happen on his Stratum nodes), but profits are distributed to everyone (both getwork and stratum workers) in the pool.

This is why namecoin income on Slush is only 10% of what it would be "normally", and agrees with what you have observed.