The signed reciepts are stored; and in case of a chargeback or transaction dispute the merchant has to provide them. Only then they are analyzed, but in practice it doesn't matter, any scrawl is considered as good as another. The only case where I have seen signature analysis being actually useful was a case where the customer was attempting to defraud the bank/merchant - claimed a stolen card but actually bought the stuff himself. But that is very rare compared to real stolen or skimmed cards, where signatures are pretty much useless in fraud prevention or shifting liability to someone else.