When a team cheats "off the field" – recruiting violations, academic scandals, that sort of thing – then wins are vacated. Off the field cheating potentially allows a team to build a better roster, but it doesn't directly affect the integrity of the competition (note the Big Ten's usage of similar language with respect to the Michigan sign stealing affair).
However, when a team cheats "on the field" – typically by knowingly using an ineligible player (see Alabama, 1993) – then wins are forfeited. On the field cheating definitely affects the integrity of the competition (and the accuracy of betting lines) and it is treated much more seriously.
What's the difference between vacating and forfeiting? When the winning team vacates a win, the losing team still retains the loss. When the winning team forfeits a win, the losing team gets credited for a win.
If Michigan's actions gave them an unfair advantage in games that they won – in other words, if they affected the integrity of the competition – then they should be forced to forfeit all of those games.