Isn't this practice... [cart reserve] ...a violation of provision 3.7.4 [1] of the RAA?
Not if the customer is logged into their account and payment information is on file, no. While a "cart hold" is a fairly decent process as described, I hear echoes of the old NSI "three month invoice float" that was the mainstay of speculation circa 1997.
Why do we need a PDP to deal with this when there is clearly sufficient room in the current contract for ICANN to deal with the practice?
Consider: "general commercial terms extended to creditworthy customers" I believe one would find that large-scale tasters do indeed have the funds on hand to theoretically pay for the registrations. Determining whether these registrations have been made on "general commercial terms extended to creditworthy customers" is going to require some information about the creditworthiness of those customers. In the enforcement you envision, then is the registrar going to have to obtain and transmit to ICANN their customer's bank statement? And if that customer has $XXM in the bank, then all's well, unless there is some other definition of "creditworthy" you have in mind. Clearly PIR was able to end tasting in .org without engaging the rest of the community in a PDP. Verisign could do precisely the same thing, and end tasting within the same short time scale. It is a mystery why these circumstances are the fault of eeeevil registrars.