Royal Mail are scrapping bikes since too many posties are falling off them and suing. It doesn't have anything to do with the bikes, people ride older postman bikes with no problems whatsoever.

The bikes I saw in the skip looked okay. They had probably been scrapped since they were the older model (like this) instead of the newer model .

EDIT: Here's an article

Although they've cited safety issues as the problem, it reads more like they want posties to use trollies so they don't have to return to the delivery office.