Was someone available over the weekend to tell them it was a non-emergency issue (such as over the phone)? If not, you have shaky ground to stand on because you don't want tenants NOT calling for emergency plumbing issues in future, do you?
Tenants have very low experience with what is emergency and what is not. Yes, my 15 year old could probably figure it out better than your tenants, but that does not negate the fact that you do not expect your tenant to make an intelligent decision when it comes to plumbing.
If you have a voice mail from them over the weekend about this, before they called the plumber, or as they were doing so, then I would pay and give warning.