A question about blind wars

Today I was watching some Poker After Dark on the italian dedicated poker channel and one of our most respected players, Flavio Ferrari Zumbini, (some good placements at the WSOP, great online grinder and excellent tournament winnings in Italy) made a pretty categoric comment about blind play. Not necessarily in pro games like the Poker After Dark, but also in online cash games.

"If the SB 3-bets after a steal, he almost always has a big hand. If the BB 3-bets after a steal, he can do it with pretty much his entire range."

So I went into my usual WTF mode for a few seconds and then put my thinking cap on.
Why?
The SB plays from the worst possible position post-flop, and this may be the main reason.
But when the SB folds, the BB plays from the worst possible position post-flop.

So my fish brain went a little step further.
If the SB 3-bets with a marginal hand and gets 4-betted by the BB, he finds himself in a pretty horrid spot.

But this still doesn't explain why the BB, after a fold from the SB, can 3-bet with a lot of marginal stuff.


Can someone please elaborate it?
Mucho thanks in advance.

Because the BB is IP v steals a large % of the time. Bada bing!

No seriously because the SB is getting a much worst price to resteal and he has another player to act behind him where as BB is the last act pre and is getting a much better price.