I agree for many acronyms the plural is logically the acronym without a 's', although if the sense is clear it doesn't really matter (and the way to make the sense clear is to avoid acronyms where possible). More irritating is the habit that some have of sticking s on the end of abbreviations of units of measurement, i.e. 1kg and 10kgs. This is a) unnecessary, since the plural is obvious b) potential confusing since s is the agreed abbreviation for second and 10kgs could be (and under the System International rules should be) read as 10 kilogram seconds. (Oh and these abbreviations are not followed by a full stop (period) either, for similar reasons)