I've always found this a bit funny.
When I'm in the office I'll pop out at lunch time and buy myself a sandwich. When I'm on site at a clients for a few hours I pop out at lunch and buy a sandwich, but this now becomes the companies responsibility?
Of course if I'm gone for an extended period and/or I'm forced by circumstance to eat expensively whilst away from my usual facilities that makes perfect sense, but coming back from a 4 hour client visit with a burger king and a receipt just doesn't quite add up to me.
Clearly there are times when the company absolutely should be paying and I don't completely agree with the point I'm making above but I've never quite managed to get my head round it.