PRESIDENT CARTER'S declaration of an "open arms" policy toward the Cubans fleeing their country is the only response he could have made consistent with both the American tradition of refuge for victims of political persecution and the plain and rising demands of public opinion. It is the right decision. The only surprise is that the administration seemed to take so long to make it. Presumably this was only because the president was occupied with other things.

Until he spoke yesterday, the American position on the Cubans was bewildering. Cubans were risking their lives to sail to Florida and boat owners were being fined $1,000 for each person coming ashore without proper entry documents. Fidel Castro's thugs were beating up Cubans applying for American visas in Havana, but the new refugee law's standard that a "refugee" have a "well-founded fear of persecution" had not been invoked; on the contrary, the visa office in Havana had been closed. There was an implicit inclination to accept the disgusting Cuban contention that the only Cubans leaving were criminals and "worms." American politicians were allowing Castro to goad them into a slanging match in which the refugee flow was made to seem strictly an American concern, thus vitiating the earlier successful effort to draw other hemispheric countries into helping deal with it. Confused officials were incorrectly suggesting that the Cubans were part of the same problem as ordinary illegal immigrants coming from Mexico.

Not all of this was straightened out yesterday by Mr. Carter. In particular, he did not indicate the possibilities the new refugee law offers for coping in an orderly way with this emergency -- one of the new law's chief sponsors, as it happens, is Sen. Edward Kennedy. It also needs to be emphasized that welcoming all Cuban comers will have predictable consequences. Depending on the numbers, the costs could rise toward $1 billion or more; this will require a supplemental appropriation. The new refugees' entry should be treated as a national responsibility, not as one to be paid for mainly by Florida and the Cuban-American community or by those who see the new arrivals as competitors for scarce jobs. Moreover, it remains for the United States, under the new refugee law, to be fair to would-be emigrants from Haiti and other places, who are no less desperate to settle in the United States but who lack the Cubans' political appeal.