First of all, no, its not unfair for Denny's to offer free breakfast during school hours. Its their promotion, they can do it however they want, and you really aren't any worse off than before, you didnt lose anything, you just weren't able to get the free promo. Furthermore, I doubt you had school at 6am. When I was in highschool, I didnt start till 730. This promo also didn't help adults with lunch breaks at all, because the lines were so out of control, no one has a lunch break long enough to be able to wait that long.
In response to your second question, of course its a marketing stunt, but that doesn't change the fact that we still got free food. Denny's is a business, and like any business they are going to do whats best for them. Why would a company sacrifice $100,000 just to do something nice? Easy, they wouldnt, unless they were donating something to some charity and could use it as a tax writeoff. Thats not the case here, they did it for one reason and one reason only. Profits. They've been losing money and losing customers. A promo like this that brings in customers out the door, hour to 2 hour waits, all the free publicity they're getting from news stations who report on how busy Denny's was today, all the publicity they're even getting on websites like this where we are discussing this promo, its all free advertising. Furthermore, if even a few of those customers come back, it was worth it for the repeat business.
Yes, marketing can be full of lies and deciept, but how many people who were getting free food today at Denny's really thought about that. If they're giving me something for nothing, I don't care why, it doesn't change anything if they're doing it to be nice or if they're doing it because they're hoping I'll come back, the bottom line is I got free food. If you look at how much they actually spend on making a grand slam, its really not all that much. They sell it for 5.99 but thats mostly profits. The extra publicity and the repeat business alone will be enough to make up for cost of this promotion.