Have you ever purchased from a store because of an advertised sale and/or a coupon in the paper (or online)? Will you go back to purchase again? Maybe, maybe not. Either way, you got something at discount and the store got its sale. If the store offers a great product (or service) and/or experience then chances are you'll go back.
I think that if you're offering something of value to the person who "Likes" your page then it's a fair trade. I'm not twisting their arm. It's still their choice to Like or not Like. But the truth is, they must have some interest to begin with in my product or service to even care about revealing and getting the discount code. Someone who comes to our FB page and has no interest in our product or service, won't Like us.
And, if we don't offer a good product or service, then it probably will be a one-time sale. But then it's up to us to offer a good product to keep customers coming back. It's simply good business.
I shop at BestBuy fairly often, but I haven't Liked them on FB. Now, if I see them offering a decent discount if I Like them, do you think I'll click that Like button? Hell yes! And I'll use that discount! I'm happy and they're happy.
As I said, I'm not twisting anyone's arm to Like us to get that discount. It's a free choice.