Sit outs in big blind

Can anyone explain this to me?
When playing in a tournament and a sit out is in the big blind, how do you play it?
Personally, if I am early to act I will raise with any thing.
In later position with a few flat callers I'll just call as well.
But after the flop the obvious move is to raise if it has not been raised already.
I do not understand the number of people who fold in this situation.
OK, so they didn't make the flop, but nor did I probably!
Just don't see the reason for flat calling then folding with a minimum raise post flop.
Anyone have any ideas on this?
Let me know
Love
Alice
www.pokersisters.com

Kem Cards

Must tell you about these if you don't know them already.
Kem playing cards are a must for home games.
They are quite expensive but are made of plastic and will last for ever.
They feel a lot different form normal card cards, and I know if you get some you will not want to go back to the usual ones.