The answer to this question is quite simple and is one that is based on something that was just discussed. If you find that your back is hurting after you have worked out on your bench, it is either due to the design of the bench (lack of cushioning, poor engineering, etc.) or to your lack of form. If it happens with multiple benches, you are either unlucky or it is your form (probably the latter). Again, you need to learn proper technique because, without it, your back can begin to feel pains that should not be persisting.