We follow Bais Hillel in most instances and even though their decisions are kulos when compared to Bais Shamai, these decisions are considered Din; not kulos. The opposite of a chumra is not necessarily a kula; it is the ikor din.
Depending on the situation, at times Rav Moshe’s p’sak was universal, at times for the USA , at times only for Manhattan which was his location, and at times only for the individual asking the s’haila.