I have clients ready to sign up but I am getting a cold feet. If I can be confident that I won't be shutdown by FB, I can go build a business out of this.
Putting aside the legal question for the moment, here's a bigger problem I think you will be facing: Of what benefit will users or advertisers have for this service?
As I current understand what you are doing, this is asking to be abused on multiple angles.
First, it only benefits advertisers if they are getting in front of relevant buyers interested in what they have to offer. So to get clicks from people not actually interested in what they have to offer will only increase advertising costs to reach the same audience.
Second, the user is just going to be deluged by offers that they really aren't interested in receiving. This means they will either de-select the company as a page they like or simply ignore the ad and get slightly irate with the irrelevant advertising.
Sorry to say, but this system is asking to be abused by click-farms. If you've got potential paying customers, figure out some other type of advertising system that will benefit both the users and advertisers, and see if it works!
Yes, this is legal and has become a huge business for companies around the world. Despite these trends, there are huge red flags in this form of marketing for companies as these 'users' will most likely not become 'live'/'real' users and will not engage with the page's content. Most companies looking for likes on Facebook will want the users to be interested with their content rather than just for the number. Engagement activity is key!
Hope this helps. I have been able to grow engagement activity for many Social Media pages and would love to help you!