On the basis of the last answers, I think my point of view was misunderstood, I'll try to re-explain it as simple as possible...
What I mean is that, at the time when LE were released, the sentences
"Any Nazgûl attack against a minion company is a detainment attack"
(which is written in the LE rulesbook) and
"Any Nazgûl creature attack against a minion company is a detainment attack"
were perfectly identical as far as the practical effect was concerned, because there was no way for minion companies to face nazgul auto-attacks, given the existing card pool. Minion companies could only face nazgul auto-attacks after the release of WH. Therefore, the word "creature" was unnecessary in that sentence.
Now, there are two possible hypotesis:
1) ICE's intention was only making clear that nazgul creatures would always attack as detainment, no matter what they're keyed (or not keyed) to, and they resorted to the most concise wording necessary, thus omitting the word "creature", simply neglecting that in the future this wording could've included auto-attacks printed on site cards which were meant to attack normally.
2) ICE's goal was actually to include nazgul auto-attacks, because they knew that they were going to let minions face nazgul auto-attacks with the release of forecoming set and/or expansions.
Proving hypotesis 2 requires that, at the time of LE release, the WH set was already designed, the WH cards' texts were already written, and ready to be printed and retailed. As far as I know, at the time of LE release, the only AS set was ready, since the release of the AS expansion was anticipated in the LE rulesbook to justify those standard modifications written on some faction card, which referred to factions which were not included in LE set, and would've only been available after the release of AS.
On the other hand, the hypothesis #1 is bolstered by the fact that over the course of the years several official erratas were necessary to change card texts or rules which turned out to be in need for corrections. This proves that ICE made mistakes. They were human beings, after all. Though one may think that a cool CCG like MECCG is a divine gift...
On the basis of a teleological interpretation, the 1st of the 2 hypothesis is more likely to be correct, as it is clear that redundancies tend to be eliminated in all sorts of communication. Actually, misunderstandings arise when the communicating parties have different sets of assumptions which they take for granted, which leads to the same messages being given different meanings. It's worth underlining that rules are always written by people on the basis of a certain purpose, they do not descend from the sky. The interpretation of a rule must take into consideration, to some extent, the purpose which is at the origin of the rule. It is not about having rules which say what they don't say, it is about having rules which say what they mean.
Now, if the generally followed ruling is treating the nazgul auto-attacks on Hog sites as detainment, that's fine to me. I'm not willing to keep on arguing about a secondary issue which seldom comes up in a game.
But I hope at least that this discussion can warn people from the harmful effects of blind interpretations without critical thinking: you find yourself in need of an UEP (i.e. something which rarely finds application in a game) in order to correct a rules inconsistency which could be officially solved by means of a better ponderated interpretation.