I agree. At least part of the problem is just how big these agencies are and just how much stuff flows through there on an hourly basis. I imagine that each photo gets just a cursory look-over, not a keen scouring designed to ferret out whether or not it's a fake. There's also a line to...
It's always been easy to fake a photograph, even with a negative. Just because Joe Average has access to the tools now, that doesn't make it a new thing -- it just makes it easier for Joe Average to do it. Any skilled darkroom guy has been able to do it since the birth of photography...