The - in front of acceleration is the crucial detail you seem to be missing.
If you left that minus out, you would indeed accelerate the Rigidbody.
However, due to the -, what is actually happening here, is rb.AddForce(rb.velocity * (acceleration * -1));.
Because of that, you get a force opposing the movement direction of the Rigidbody.
Do note that a very similar effect could be achieved by simply increasing the Rigidbody's drag value (which acts similar to air resistance).