Because for them it was entertainment.
Historically, public executions were spectacles often witnessed by huge crowds
An Act of Parliament ended public hangings in Britain in 1868, but it took America nearly seventy more years to follow suit. In colonial times, says one account,
“Crowds of thirty thousand or more were expected to be at high profile hangings; viewing stands had to be erected to contain the throngs. Often chaos erupted out of these bloodthirsty crowds — a viewing stand collapsed at an execution in Cooperstown, New York, and killed two people.
The Last Public Execution in America, says a book with that name, took place in Owensboro, Kentucky, on August 14, 1936, “when some 20,000 men, women, and children witnessed it.”
The script indicates the same intention...a holiday/entertaining atmosphre...
The CROWD is in a carnival mood; HOT CORN GIRLS, STREET VENDORS,
"HOKEY-POKEY" (i.e., ice cream) MERCHANTS, even BUSKERS, all add to the
Regarding the sale of the prisoner's belongings, it's less clear. The implication is that the proceeds are retained by the persons actually conducting the execitions...since the bidding is started/controlled by the Assistant Hnagman in the script.
What am I bid for this coat? A coat of some wear but excellent cut...
containing a rather remarkable pocket silk...
It's possible that the proceeds are then passed to the prisoners heirs but given the nature of the movie and the times I would suspect that this is not the case.