In Alberta, Canada, anything BEHIND the driver can be up to limo (5% light transmittance) tint, but the front drivers' and passengers windows can not have any aftermarket film at all. I think my '04 has a very light factory tint on the front sides, and that's OK. I know some RCMP's and one reason they give for this rule up here is that the plastic tinting film actually holds the window together, so it wouldn't shatter (which is good that you won't get hit by the safety glass), but not so good if you had to break a window to get out. I guess the other factor, though not as big a deal up here it seems, is the being able to see in thing.
All that said, I also know that some cops will hassle you just because, depending on the day they're having. If you respect them, they'll (usually) be fair with you.