Why do adwords send vouchers knowing i cant use them?

Ok I have been with adwords since 2006 and generally I don't use them too much as I find them overpriced. But every now and again they send me voucher codes knowing I can't use them. Want $100 ??? sure just put in this code that doesn't work for your account!

Well I have $100 i cant use can anyone else put use to it?

Just sell it on fiverr for $5

lol.. probably could if anyone can get it to work. How does it link to an account?