"How much would you charge for me to pee and poop on you?" Very professional response! Just like some little kid! You have a multi tenant building and can not charge the tenants unless you can pin point the cause and provide proof which tenant caused the blockage. How would you provide proof in court that all the tenants caused the blockage and thereforth should be charged equally? You should be thankful the tenants told you before it became a major problem. If you try to charge them, all you will get is no response from the tenant next time until it's a major mess. Might even get notice they are moving.
"You have to relize that we are all used to putting up with tenant's sh*t..." Speak for yourself as many landlords do not have problems with tenants like you do. Perhaps better screening would prevent you having to put up with so much tenant's Sh*t.
You should have had a Plumber look into the cause and fixed the problem. All you did was clear the line without knowing what the cause was. Very Professional for an un licensed plumbler and then to try to charge the tenants $300.
Perhaps you should have a Profeesional Plumbler look at the line to see if it's not crushed or has a dip in it that causes the problem.