Getting off scot-free refers to someone getting away without payment, either monetary or otherwise. In fact in modern usage, it often refers to suspects who are not convicted of a crime. If a person feels that the suspect should have been convicted, he might say, “That guy is getting off scot-free.” Often the term is confused with the frugality that is occasionally attributed to the Scottish. Actually, the term scot predates old Gaelic that would have been spoken by the Scots. In fact the word has its origins in Scandinavian language and probably descends to the English language via Anglo-Saxon.
To get off scot-free implies evasion, purposeful or otherwise of the dues one is supposed to pay. It is frequently an annoyance to others if people duck out of paying what everyone else must pay. This is especially the case if the scot of a bar tab is avoided, since everyone else will have to pay extra to make up for the person getting off scot-free.