If there are 50 cookies all the same on one tray, and 5 different cookies on a smaller tray, would you not wonder what was different about the other cookies? If you thought , "cool, different cookies" and just took one, when there are clearly more than five people at the party, then I call that thoughtless and selfish.
I disagree - I think most people appreciate novelty - if the 5 cookies look like a special treat of some sort (that there may only be 5 of because they are so good, people already ate the other 45) wouldn't you want to try one? As someone who has planned events, it is just a bad idea to expect guests surveying a buffet to follow the same set of unwritten rules you had in your mind. Either put the vegetarian stuff in another location, or have enough extra veggie burgers so that if everyone wants one you can make more.
I suppose that I was just brought up with very strict rules regarding communal eating, which were as follows:
1) Look at the dish being served. Divide it by the number of people there. Take that amount or less until you are sure everybody has had their first helping.
2) Don't take the last of anything without offering it to others first.
3) Take only what you are sure you will eat. If you want to try something new, take a tiny helping to taste.
4) If you touch it, you take it.
So, if there were 5 cookies (and they were presented in such a way as to make it clear that there were only ever 5 of that variety, and not 5 left from a batch of 50), then even if they looked delicious, politeness would forbid me from taking one because of rules 1 and 4. A whole cookie would be too much to take for just me, and there's no way to break a piece off without leaving an unappetizing piece that I've touched.
That's quite apart from any consideration as to why a host would serve 50 of one kind and only 5 of another, which is surely the first thought that would occur to me, leading me to conclude that there was something else at play here.