One of the (often unstated) assumptions in this form of logical puzzles is that when it's a given in the puzzle that someone says something (like the guru's announcement in this puzzle), then you just take it as read that they said it, don't care about their motivation for saying it, and that the only information it carries is literally the content of the message.
For a simple example... say I have a deck of cards, and I draw a card at random. Now, say if I have this habit where if I draw an ace, I'll tell you "I have an ace", but otherwise I'll say what colour it is... so I draw my card, look at it, and say "It's a red card". Now, with that information, you'd know it's a red non-ace card. But without that information, if this were in a logical puzzle where all the puzzle says is "I draw a card and say it is a red card", then all you can take from it is the context of the message, and assume that all 26 red cards are equally likely.
For a more concrete example, take the Boy/Girl paradox
, often worded as "A parent tells you they have two children, and at least one of the children is a boy. What is the chance that both are boys?"... Now, you can take the stance of "in what circumstances would they make that statement?"... if they have two boys, then they would make that statement, but if they had one boy and one girl, then they'd only make that statement half the time (the other half they'd instead make "... and at least one of the children is a girl"). This then changes the probability calculations by adding extra weight to the two-boys case. However, the standard logic-puzzle reading of that phrasing is that the message contains exactly the information in the actual message - that at least one of the two children is a boy.
Normally when I tell the "Boy/Girl paradox" puzzle, I word it to take the "motivation" angle out... "I have a friend who has two children. I asked them 'do you have a son?' and they answered 'yes'"... which removes the motivation question by making it clear that the friend would respond identically in any situation where the answer to the question is "yes". The decision to ask about sons rather than daughters was made by me, rather than them, and couldn't be influenced by whether they actually have sons or daughters, because I don't know that.
I suppose you could do a similar rewording of this puzzle... instead of having the guru make their announcement spontaneously, say the guru decides that after a lifetime of silence, they will truthfully answer a single question. A blind traveller (who happens to also have green eyes, and, if it helps, is not necessarily a perfect logician) comes up to the guru and asks "is there anyone on the island with blue eyes", to which the guru answers "Yes".
Now the decision to ask about blue eyes rather than brown eyes is still arbitrary, but you can't make any inference from the choice, since the person who made the choice is blind anyway...