Why was Light only targeting Japanese criminals when he was in the US?
Notice the "keep killing Japanese criminals" part; this seems to suggest that (mostly) Japanese criminals were being killed by Kira during the time right before this incident. L had already previously established that Kira was operating from Japan on live TV. Any aberrations in killing behavior during Light's time in the States would appear highly suspicious to Near, who already distrusted the Japanese Kira investigation team at the time. Light could just be getting paranoid and probably wanted to reinforce the idea that Kira is operating from Japan during his visit.