Disabling dhcp does not bring the connection down; it kills the dhcp client, meaning any further IP changes won't happen. That's as intended.
edit: I tested the button in 6.4.1 - killing works fine, but it refuses to start it for a connection with an existing IP. This is also as intended, it shouldn't break an interface that may have a manual IP. I agree it's confusing for an user clicking it though.