It IS most definately illegal to have any additional tint applied to your front windows:
http://www.pentagonglasstech.com/the-law.php
The legal minimum is 70% for front side windows, so if standard windows are 70% then you can't put anything on them, apart from clear security film.
The standard glass is closer to 80% I beleive, but security film efects the light transmition as it's not completely transparent. But it's only 95%VLT so you're fine with it.