Presumably on the same grounds that it is acceptable to form plurals of numbers and letters generally with apostrophes. The common example is "mind your p's and q's". As I say, personally I generally prefer formations such as CDs and 1970s, but as the alternatives with apostrophes are accepted in formal style manuals they can hardly be said to be wrong. And I definitely agree with King Pellinor that when writing in all-capitals, the apostrophe is very useful for clarity, and I would not hesitate to use it there. While the apostophe is commonly used either to indicate 'missing letters' or to form a possessive, there are acceptable cases where it may form a plural. Plurals of abbreviations (especially I assume initialisms and acronyms) is one case; also certain very short words may (although they don't have to) use apostrophes for plurals, such as "do's" - even the OED accepts these as valid options! Just because you (or I) prefer something doesn't necessarily mean it is the only right way.