You're right... it doesn't move... until you hit something at 40+... putting nuts under the car makes it so the seat will sit still... but those nuts can rip through, since you're putting bolts into/through SHEETMETAL.. rather than the REINFORCED AREA that was actually meant for that specific purpose.
It's illegal because you're defeating a standard safety feature, which makes your vehicle officially unsafe for use on public highways... this isn't a state thing, it's federal, since the Federal Highway Commission works with car manufacturers to help develop new safety features, then put them into use and enforce their use.
Just because you've done it before doesn't make it legal... I've street raced for thousands of dollars through-out my career... it's still illegal, and had I been caught, I'd have lost my license, just the same. (Actually did happen, hence why I don't do it anymore.)