It's probably legal ... it's outdoors, right?
What I don't understand is what makes them pigs. Them dumping garbage by the garbage receptacle? If the landlord is offering trash service, and they're only throwing away things from their apartment, then the landlord should cover that. What is an apartment dweller supposed to do with their old couch that they want to throw away if they don't put it in the trash? If he doesn't want to cover it, he should put a provision in the lease, otherwise he can't charge people extra.
No, I live in an apartment complex of pigs, some people will leave garbage in the hallways and stairwells instead of hauling it out. Count your blessings.
**ADDED**
In response to the section where you replied to me ... I'm not a landlord, so I don't know what they have to pay or what arrangements are made for trash pickup. I live in a 100 unit apartment building and some of the apartments are quite large. People leave furniture out by the dumpster all the time. And when the garbage truck comes they throw the furniture and broken TVs right in the back of the truck where the rest of the garbage goes. That's why I'm wondering why you said they " ... ACTUALLY require a different way of disposal." When they actually don't. There's not recycling programs for old furniture, that I'm aware of.
I've lived in other apartments too ... there was never an issue over the disposal of old furniture. The landlords never had a problem with it.
Has anyone tried posting notices around or handing out fliers that tell the tenants they're not supposed to do that? Or maybe your landlord should tell people that large item service costs more, and that they have to make arrangements with the management for disposal of large or unusual items.