Earlier quoted context omitted.
doesn't any rule decrease password entropy?
It's ironic that allowing low-entropy passwords (for example, one-character) can actually increase the available entropy. For example, if you set a minimum password length of six characters, an attacker doesn't even need to bother going through all of the 1 through 5 character combinations. The flip side of the coin is that, obviously, allowing low-entropy passwords will inevitably mean that some users will actually…
Well, the theoretical entropy, but not the empirical.
You could argue that ruling out "12345678" and "password" and "Password!" reduces the "available entropy" and makes things less secure (after all, your random password generator might just randomly have generated "12345678"), but in practice quite obviously it makes things more secure.