The full, original label is "soda pop," in which "soda" is the adjective and "pop" the noun. Basic English grammar dictates that if you're going to abbreviate an adjectivally modified noun, it should be abbreviated to the noun, not the adjective. For example, if you were going to ask someone for "cold water," but didn't want to use two words, you wouldn't say "I'd like some cold." You would say, "I'd like some water." Therefore, the only grammatically correct label (other than "soda pop") is "pop."