It's better when there are no or not enough "slots" to give away. Think of it as a problem in your hotel room when the hotel is booked solid for the night. They can't move you to another room so they offer you a free night.
Yeah, they may lose money down the road on that free night but they can't really do anything for you in there here-and-now that gives you what you want.
In the case of a ride like FOP, I suppose they could just give everyone impacted an anytime fastpass and then watch the fastpass line balloon to a 4 hour wait. What does that accomplish? It royally angers the people with fastpasses for that timeframe and does nothing to really help the people you gave the "extra" fastpasses to.
Giving people tickets to come back seems like a huge gesture given the price guests pay to get in but it is really only a potential opportunity cost for Disney. The only way they lose out is if someone is using that ticket at a point they would have gone anyway and paid full price or on a day when the park has to close for capacity and they couldn't sell someone a ticket because that person got in for free.
Even if they lose out on the chance to sell that person a ticket for the day they decide to use it because having the free ticket wasn't the main motivation to go on that particular day, they still stand a good chance of making better-than-otherwise money on that guest in terms of food and merchandise sales since many people are inclined to "splurge" more when they feel they have gotten a deal - like buying concessions at a movie theater when going on a free ticket when you might otherwise not have if you'd paid full price to get in.
Would that hit break-even on a ticket price? Maybe. Maybe not but it isn't necessarily as bad as saying they lost $100 in that situation.
Free tickets are probably one of the easiest things for Disney to offer as long as they don't do it all the time for any and every reason and it feels to the guest like a major compensation when in reality, you just walked out of there without Disney refunding you anything, without giving you anything that it really cost them anything to produce (like food or merchandise), and it means you're coming back with the likelihood of spending more money with them in the future.