I think the real problem with push daggers is that they're:
1) Unusual and sinister looking.
2) Have very little practical purpose except hand to hand combat.
3) Are often illegal.
If I'm on the job and I get stopped by a supervisor or a customer, I'm going to have trouble explaining a push dagger. A folder with a thumb stud on the other hand is no big deal. "It's just a pocket knife. I use it to open boxes and cut cord and stuff." I won't lose my job or get written up for having a pocket knife.
If I get stopped by a cop, same thing. "I use it for opening boxes, cleaning fish, etc..." I'm much less likely to get the full gestapo treatment if I'm carrying a knife with a thumb stud.
The fact that a knife is a utilitarian tool and not just a weapon means that most of us can come up with a good reason for carrying one aside from self defense and that's important to some people. If you are carrying a weapon for self defense however, it seems very desirable to me that that weapon be somewhat socially acceptable. Properly concealed handguns and pepper spray are two examples of such weapons. People know why you're carrying them, but if they were shown to a jury they wouldn't look unusual. A push dagger on the other hand would just look bad in court.
I've got a gut feeling that upon finding a push dagger cops would suddenly want to pat you down, search your car and run a background check. If you used it for defense, a prosecutor would be far more likely to put you on trial and a jury would be far more likely to convict you.