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The vouchers are very real and follow the same line as the offers that other types of promotion take.
Many people, inspite of lots of column inches, will be skeptical of PPC. By offering a new advertiser the chance to make mistakes with the providers money there is more likelihood that they will remain as an advertiser.
You could liken it to a money back guarantee, where people would feel comfortable.
It is all over the internet, but clearly the fact you need to put a credit card in and the voucher is for a fairly nominal amount is notgoing to escalate it into a "scam".
If your not advertising on PPC it's a good way to get some support along the way.