Merchants are required to pay a business fee. Why are people permited to drag their junk into their yard at no cost. Either start making people pay a buisness fee for selling their junk or stop charging legit businesses a fee for selling their goods.
You seem to have answered your own question. The answer is because it is their property. Taxes have already been collected on the items once, and the resell value is rarely more than what was previosly paid for it. What might be junk to you, might be someone else's treasure. I've found several great finds at yardsales, most items coming from upscale neighborhoods.