Yes, it is a legit Government website, and it functions just as described here.

They are willing to sell you the bonds via your credit card without a commission or any fee at all.

It's not a scam. It's worth it to them. If you keep the bonds then everybody comes out ahead. If you sell them in less than 5 years, you eat (just over) 1 percent interest. Probably the Feds have arranged to pay Visa et al no more than that for the merchant's fee.

It's good to be skeptical when the math doesn't work out, but in this case the numbers do make sense.

If it suits your investment needs, keep the bonds. If it doesn't, cash them in after 6 months and the mileage will have cost you just over a peny, which is a fine deal.