Tires: Rear tires replacement, rear tires, correct direction

Question
I wanted to replace two tires for my car. Costco said they only install two new tires in the rear and choose two good condition tires for front tires. I thought the front tires always to be the better tires. In this case, new tires. Do you know why? Is there any law in California about this? Costco said it is the law.
Thanks in advance.

Answer
Jery,

I don't think it is a law anywhere, but it is the recommended practice.

Imagine it is raining,.  We know that as tires wear they lose hydroplaning resistance, (snow traction, too!)

So if you lose traction on the front, once the vehicle slows down, you'll get the traction back, and you'll be in a postion to steer around whatever is ahead.

But if you lose traction on the rear, you spin out.  And no matter what happens after that, you'll be facing the wrong direction.

It is better to be facing the correct direction, but it would be better still, if you had 4 new tires, and that's where frequent rotations come in.