You have missed a big part of the picture.

With mail in rebates, the store sales figures aren't
decreased as they would be with the instant reduction.
Localities like it because they get the full sales tax
amount.

In most cases the store doesn't pay the rebate, the parent
company or manufacturer does.


Barry Watzman wrote:
An "instant rebate" isn't a rebate at all, it's just a sale price. And
no doubt that it's better than a mail-in rebate of the same amount, but
you will NEVER find "instant rebates" that are the equivalent of the
larger mail-in rebates. The rebate "system" depends on the fact that
only a minority of buyers will ever apply for the rebate.