Floating down to Camelot

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.