1997 was a very special situation. It wasn't(and never looked like, the available shots absolutely left no shadow of a doubt, unlike, for instance, Adelaide 1994.) a defensive move going over the limit, it was a plain attempt at turning into the other car, and causing as much damage as possible to it; he never had the opportunity to make it look like a racing incident, like for instance Adelaide 1994. It was the first time in 3 years Schumacher pulled a dirty trick to be champion, and most observers blamed the FIA for taking no action in 1994, so, failing to do so 2 times in 3 years definitely would've looked bad.
Furthermore, the penalty did not affect the outcome of the championship. Villeneuve would've been champion anyway. And no one really cares about the runner-up spot, ask the average Joe F1 fan who was runner-up in 1997, I doubt he would answer correctly. However, the FIA doesn't like applying penalties that decide the championship a few weeks after the finish of the last race - it doesn't look good for the sport(cf 1990, 1994 and 2007). If Schumacher did manage to take Villeneuve out that day, it would've been rather -interresting- to see how the FIA would've reacted.
Last thing, may I point out that the marshalls(although championship disqualifications would be decided by the FIA's Great Council AFAIK) are usually much more permissive since last year. That's the way Todt wants to go - let the drivers police themselves.
