rusty frames common on older Silverados?

I've been looking at older Silverado 2500s. I've ran into 2 trucks with rusty frames and I don't mean surface rust, I mean scaly rust. The first truck was a 2003. It sat on a local lot for a while and when I came back to look at it, the dealer sprayed the frame with undercoating and it sold soon after. The 2001 I looked at today had moderate rust on the entire frame and it looked worse near the back cross-supports. It looked like that dealer also replace the rear bumper and it appeared to have a new, but undersized 5,000 hitch on it. Are rusty frames common with the early 2000 GMs or could they have just come from northern states that salt their roads more often?