well firstly they'll ask you, secondly they'll be able to see on the broadcast monitors and scopes, thirdly they can check the metadata, and fourth they can put it through an HD analyzer (which any handycam will 100% fail).
Exactly, thats the key. As I said above, If you have a unique event to film (like that fire) and you are the only person to have filmed it, then it doesn't matter so much about quality. Problem is finding such an event and being the only one filming... Remember we all have cameras in our pockets now, something that may not have been around when you filmed that fire.
Out if interest, how much did you sell it for?
That then leaves you with the kind of stock footage that I thought you were talking about... The out of focus school children on stories about schools, film of a blocked motorway, shots of kids in hoods on the street, etc. And for all that tat, you need proper kit. Remember they've already got all those shots, so why would they need to pay for yours?