The laws on bald tires can vary from state to state but in some states it is actually illegal to drive with bald tires.
The state of California has cops watching out for people driving cars with bald tires and if they see you have bald tires on your vehicle they will stop you and can write you a ticket.
The ticket can be as much as $150.00 or more and it's just cheaper and safer to get new tires than deal with the police and pay the ticket.
Bald tires are a safety issue because they can't grip the road properly and when they go bald the steel belt of the tire begins to come through.
When that happens it's just a matter of time before the tire comes apart and causes you to roll the vehicle and possibly get killed as a result.
I never run my tires bald and if they get to that point I change them out for my safety and my families safety as well as others safety around me.
I have seen some cops driving on bald tires too though and they really should change them out.