I agree with your best efforts, but the problem is that even though you select guest checkout, the vendor most likely keeps your CC information at least temporarily and possibly permanently. I have a small business that does not do online sales (it would not make sense), so this could not happen to us.
We do accept credit cards, but the card must be present and be inserted into our CC terminal (We cannot key the numbers in manually). It is connected to the Internet, but it does not store any credit card information locally. When a charge is made the CC information is transmitted to the processor via an encrypted connection. From that point on the CC information is gone and all we have is a printout with the name and last 4 digits of the card.
Because our terminal is connected to the Internet we have to complete PCI Compliance every year and we have to agree that our public IP address is subject to penetration testing at anytime by the group performing the PCI Compliance. It does not matter than we do not store card information on a computer or on paper. There is a questionnaire that is a part of the Compliance that is so ridiculously technical that virtually no one could answer the questions with certainty. I call the processor for help answering the questions because even though they are "yes" or "no" they are more like "yes, maybe" or "no, maybe".
All that sounds great in terms of a basic level of security, however you can opt out of doing any of it by agreeing to pay slightly more to process your cards. The price difference is minuscule and some companies probably just pay more to avoid the rules.