If we diagram the conditional statement, we get the following
Zam-> Lam Lpm-> Zpm
What you find is that this looks as if it is saying that Z and L are always together. However, upon closer inspection, we could have the necessary condition occur without the sufficient condition, as with any other regular conditional statement. This would be Lam and Z not in the am (PM). This is what we have in this hypothetical and it doesn't violate the original rule. You can think of it in the following way: we have Zpm which does not trigger any rule.
I hope this helps. Please let me know if you have any other questions.