Not so much a crime, more an attack of sour grapes

There’s a lot of talk about the EU ‘punishing’ Britain for daring to vote to leave it. Could someone please remind me when membership of the EU became obligatory and Britain’s decision to quit became a crime requiring punishment?
    Matters arising include the question of how the EU could punish Britain without inflicting at least equal, and probably greater, damage upon itself. After all, we’re talking about a nation which  is actually putting cash into the budget and which has a huge trade deficit with the EU.