[Pollster1]

"Romney earns the backing of 36% of the state’s registered Republicans, a slight edge over the 34% who prefer Gingrich." Why are they polling registered voters instead of likely voters?

A gold star for you - that is the key question. Newt's voters are a lot more excited than Romney's tepid supporters. Even those who don't love Newt hate Romney so much that they will be there. To be fair to CNN, they normally go with registered voters, so this wasn't a particularly pro-Romney decision (and to be honest they do that because registered voters are always more liberal than likely voters, so this was a pro-liberalism decision).

[Praxeologue]

Why are they polling registered voters instead of likely voters?

They included 369 Republican-registered and likely voters. Out of 1,500+ polled, they only included the 369 subset that was both registered and likely.