I asked our city attorney that very question... and he explained it this way:

Dumpster diving:
The county had to outlaw dumpster diving because people were hurting themselves and then suing the county for damages. So they passed a law and put up notification signs. Even that wasn't enough... a guy STILL got hurt and STILL won a lawsuit for damages. So now there are no public dumpsters in our county at all. All private dumpsters are subject to trespassing law whether you're taking stuff or leaving stuff. You have to take big items to the dump yourself and pay a fee per pound for anything that you dump. And you get fined if they catch you taking anything out of the dump.

Taking junk from the curb:
The police department asked for this ordinance because they had tons of trouble with people stealing things out of the front yard and claiming "I thought it was thrown out for the trash man!" His most notable story was of the guys who got caught loading up a brand new gas grill... one of those $3000 outdoor kitchen types... still in the boxes. They claimed "We thought it was out for the trash man!"