What is being 'indirectly' paid for is the Beckham's image. Not only David but Posh aswell. As I understand that they are very popular across the pond.
You're right to say he's over the hill for the UK Prem but I think he's more than adequate for the US as Football (Soccer

) is still in it's infancy there.
As for being a gamble, your right. However, I think it's a good one as he is an international name/face/star etc. and as a package together they are probably one of the most branded/labelled/photographed couples in the world.
It's still a lot of money - £43 per min.