I actually think this is a legitimate complaint and I believe Delta will address it. Hopefully sooner than later.
As for why people keep buying them, if you look at the reviews online, most have 4.0 to 4.5 stars. Home Depot 4.5, Lowes 4.2 - 4.2, Menards 4.5, Wood Magazine 4.0. One site suggests it is a good saw for the price even though they address the issue of the knocking.
I don't knock people for buying it (as it appears to be an easy fix) but I do think they should provide negative feedback as most companies will review these. If they continue to get 3, 4, and 5 stars, the company will continue to sell them as is.