That "hack" has the nasty characteristic of probably making everybody lose, maybe more than the $200 the guy was getting.
I imagine shipping was not free for the government, which also probably paid the credit card operator a fee. The credit card operator is losing money by paying rewards on a bogus transaction.
Nobody loses, except people who object to the subtle inflation by seigniorage. The Federal Reserve can control that anyway.
Here's someone who bought $15,000 in coins: http://advancedriskology.com/the-day-i-bought-15000/
5% is basically always either category-limited (Gas, sometimes restaurants) or limited to a trivial (<$5k/yr) spend.
I personally do the American Express Starwood card, since I value Starwood points (1 per $1 in general, and some other bonuses for hotels, gas, etc.) at about $0.025-$0.04. You can also convert SPG points into FF miles at a rate better than unity.
Back in the day, paying for space segment ($200k/mo) on a business credit card meant never having to pay for a hotel suite or airline ticket.
The Chase Amazon card is also 3% on Amazon purchases, which basically means 3% on every mailable product in the world. I think it also gets you 3% on Amazon Web Services purchases...I doubt Dropbox or Netflix pay their bills with that card, but it would be amazing :)