It's a style difference. The Guardian is a British paper. In British English, acronyms are usually written in title case, rather than all-caps as they are in American English.
"our style is to use lower case with an initial cap for acronyms, where you would normally pronounce the set of letters as a word (eg Aids, Farc, Eta, Nafta, Nasa, Opec, Apec)."
No, it's because we see them as mostly the same thing. It's only when we no longer really care about what it stands for, and that it's become an actual word (rather than a name), where we stop capitalizing it, and then we don't even use titlecase. Examples are "laser" and "snafu".
NASA isn't a word, it's a proper name, and the agency itself uses all-caps when referring to itself. The proper spelling is therefore "NASA". It's just like this for any proper name: there's a correct way to spell and capitalize it, and any other way is wrong. An example here would be any company which uses CamelCase in their name, and officially spells their company name that way. If you decide you don't like camelcase and only use titlecase, then you're spelling their name wrong; you don't get to decide how to spell someone else's name.
This is just another example of British English being wrong-headed, just as they do with referring to corporate entities with plural. American English isn't perfect by any means, but modern-day British English seems to go to great lengths to be different for no good reason.
