I guess this is a little 'wrong', but as long as they know, and the bills are paid on time, there really isn't an issue. I got my Grandparents on to Membership Rewards a long time ago, and they could care less about points. They were using a useless credit card for all of their purchases, and I threw the thought by them of using a point-accruing CC. There's nothing wrong with it - old people really genuinely like nothing more than pleasing their Grandchildren (ideally). They would be thrilled at the opportunity to help them fly in CX F getting drunk of Krug while wearing ridiculous Shanghai Tang pajamas that no one would otherwise wear.

You just need to of course maintain financial prudence and avoid referral links from bloggers, as their credit score is (maybe) as important as your incessant search for availability on NH F.