They're saying that it's pretty silly to compare buying ice cream to being mugged. If it's too expensive, don't buy the ice cream. The price of ice cream isn't an invariant, if nobody buys it the price will decrease. If nobody has a problem paying $5/scoop for ice cream, that's what the price is going to be.
There's nothing silly about it. It's just a metaphor. It's more incredulous that people have a problem with figurative speech like this than the comparison itself.
Sometimes prices are inelastic after you've stood in line wanting ice cream. You have to price in human emotion. Next time he'll probably find a cheaper place.