Why is 'yes' the answer?

This isn't so much a puzzle as a mystifying question that I can't stop thinking about.

When asked a question with a choice, 'is it legal or illegal?', someone for some reason answered 'yes'.

Somebody else commented that you cannot answer a choice with 'yes', but that someone wrote 'More formally, the answer is "Yes, it is either legal or illegal, and you didn't ask which."'

I thought 'or' was used to give an alternative, so how can 'yes' be the answer to a choice you're given?

By the way, the question can be re-worded any way you like: 'are roses red or green?', 'should we go out or stay in?', etc.

In my opinion, his answer is wrong. He has misread the question and answered 'is it [legal or illegal]?' (taking those three words as a whole, or a noun if you will), instead of 'is it [legal] or [illegal]?' (the question as a choice).
I hope you can put my mind at rest. :-)

I think your question was misunderstood, as "or" does not automatically predetermine exclusive choice between one of two options, but can indicate a list of possible choices. Your question, phrased as it was could equally indicate a desire to affirm those two prticular attributes as possible choices i.e. Is this particular thing able to be categorised as legal or illegal?  such as the example you followed it with.  Are roses red or green (as opposed to classified by smells - pleasant, etc.).  It is usual however for native speakers to give opposing options in the manner in which you formed your question however, so that is perhaps where you came into a confused response.  A question of common as opposed to book grammar.