The problem with calling the 20" 'low-end' is that it is only the low-end of the iMac Apple still positions its brand at the high end of the market as a whole, so a 'low-end' iMac should still be on the higher end of the spectrum overall. And unfortunately these screens are absolute crap.
Possibly, but this is nothing new. Apple have always been relatively expensive for their tech specs and low end iMacs have nearly always had these crap TN displays. I would accept this argument 100% if this was the first time Apple had fitted a TN display in their low end iMacs and if their prices hadn't dropped as a result.