NEW YORK — Relief was the main sentiment of travelers who planned to pass through John F. Kennedy International Airport in New York when security workers there canceled the threat of a Christmastime strike. Yet two troubling questions linger. The first involves the airport workers’ specific grievances. They complained of making only $8 an hour. Why so low, you wonder, especially compared with what union workers in security-related jobs get. The guards, hired by the contractor Air Serv Corp., were so angry that one of them even expressed a warning that those in safety fields often don’t make explicit: that a labor action could compromise the public’s security. “They will be completely unsafe,” said the guard, Prince Jackson.