I can't refute this claim any more than you can back it up, so lets not waste our fingers on an argument about it. I agree with everything else you said, though.
1.) None of the iPod's except the touch have WiFi or any internet connectivity. Can't stream cloud to device if theres no connection.
2.) Infrastructure. Consider for a second that albums routinely push 100+MB, and think about pushing that over 3G/LTE. Telcos will make a fuss, etc. Yeah you can force downloading only over WiFi, but then again whats the point? iTunes already has WiFi sync, and to most people that would be the exact same thing.
3.) iTunes is already there and most people already have it installed. If you don't like it, you can also use the online store on the phone.
What it starts boiling down to, is Apple is going to spend x-y million dollars building an online marketplace for some people who don't want to install iTunes.
What does an online storefront do for consumers that iTunes doesn't?
Trying to explain to my parents how to get a music file from their browser downloads folder into iTunes would be challenging to say the least. However saying just click this button to Buy and it will be in your Recently Added playlist is much easier.