That concern you have is nothing to worry about. If you buy a domestic ticket for USA and read the fine print it will most likely say the same thing. I have bought from Turkish Airlines many times and was never asked to show credit card I purchased with.
The only consideration for the credit card is if you will use automatic check-in and boarding pass machine at airport - the machine will prompt you to insert your card to print out the boarding pass. Your in-laws should bring e-ticket and go to check-in desk at airport where they will not be asked for credit card.
All your travels need is a valid passport, a travel visa to country they are traveling to (if one is needed based on their home country), and, a copy of the e-ticket (which you were emailed when you made the purchase) that shows your travlers names for which you bought a ticket.
davcamphttp://web.me.com/dav_camp/Istanbul_Tra ... Guide.html