i in-putted my credit card details then straight after it asked me for a voucher code which i entered
i then went back to remove my credit card details (they dont need them as its a free service) and i couldn't, have emailed them to complain and asked them to remove the details from my profile
My theory (and it is just that) is that the credit card number is required to validate the Lightbox user account to prevent misuse or multiple accounts per user. Or it could be to verify your 18+ status or country of residence (Lightbox is only for use in NZ).
I note that even if you only ever want to download free apps from Google Play, every Google Play user must still provide valid credit card details AFAIK.
I have been on a free Lightbox trial for nearly a year, but I also had to enter my credit card details when taking that offer up, however I have never been charged anything, and my status has now changed to show that I am a permanent 'free' user. I'm not really worried that I can't remove my credit card details.
I am sure the good people from Lightbox will provide an official line on the issue though.