ProCD is about the law of contract, not the law of software. Payment preceding the revelation of full terms is common for air transportation, insurance, and many other endeavors. Practical considerations support allowing vendors to enclose the full legal terms with their products. Cashiers cannot be expected to read legal documents to customers before ringing up sales. Others would hang up in a rage over the waste of their time.
Writing provides benefits for both sides of commercial transactions.
Competent adults are bound by such documents, read or unread. For what little it is worth, we add that the box from Gateway was crammed with software. The computer came with an operating system, without which it was useful only as a boat anchor. Gateway also included many application programs. But of course ProCD had not completed performance with delivery of the box, and neither had Gateway.