I am a merchant myself... and yes it is definitely against their rules.

The 1.9% you see is usually for non-reward debit cards. No way you will ever find a reward card that rings up less than 2.5%.

In fact, my Barclays US Airways Mastercard and a lot of others I have are 4% (non-swiped).

You would be better off using PayPal where they have a flat rate with any type of card you use (however, it is still more expensive than just buying miles from the airlines) and also against their rules.