Personal comments under the spoiler.
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Obviously, this is a kind of game where the most efficient strategy is to keep the minimum possible offer.
It's just like playing the prisoner dilemma.
Everyone has convenience confessing (choosing 0) but everybody also knows that if everyone put 0 into pot, nobody will win anything.
On the otherside, this choice will grant no loss.
The best cooperative choice is to put 20 but, if everyone puts 20, I will earn much more reducing my bet, up to 0.
This because any played coin will have revenue $1/2$, whilst any non-played coin has revenue $1$.
This is the same problem known as "free riding".
Whereas possible, anyone takes advantage dodging taxes, if everybody else pays.
Of course, this is one of the reasons why the state needs to attend.
This is my personal opinion and spin of the problem, given my economical point of view.
I will wait for Fioravante's explanations, which leave me nosey.
EDIT: I put comments under spoiler tag.