If you were to ask a random member of Overwatch's audience what their opinions were regarding Blizzard's microtransaction model for the game, it is likely that their answer would be fairly positive. After all, Blizzard has given no reason for most of us to doubt their claims that all future content for Overwatch will be free thanks to all the people who buy their Loot Boxes, especially with the release of Ana, the Summer Games event, and the recent Halloween event. That being said, anyone who played Overwatch during their Summer Games event would likely point out that not everyone was pleased with how Blizzard handled the distribution of Summer Games items.
During Overwatch's Summer Games event, every Loot Box that you opened had a guaranteed Summer Games item in it. Unfortunately, it was also fairly likely that you would get a duplicate of a Summer Games item that you already received, which would basically give you nothing in return. Usually, this wouldn't be a big deal, if it weren't for the fact that the Summer Games items were only going to be available for a limited time, and that people were paying real money for (in this case) a very real chance at receiving nothing of value (at least in context of the event). It is the rough equivalent of going to a restaurant, putting on a blindfold, slapping some money on a counter, opening your mouth, and saying "surprise me," only to receive store credit for a furniture store.
For Overwatch's Halloween event, however, Blizzard has apparently had a change of heart in regards to their handling of themed items; while you are still at the mercy of the game in regards to getting duplicate items, you can now use the currency that you get from duplicate items to (eventually) buy any Halloween item of your choice. Granted, the Halloween items are three times the price of any other comparable item (which is actually rather fair, all things considered), but at least you can spend real money without the risk of never getting a specific item that you want for a specific Hero. Naturally, most people are praising this decision, claiming that Blizzard is listening to their fans and so on, but it is entirely possible that Blizzard isn't doing this entirely out of the goodness of their collective hearts.
Take the Summer Games event example: if you had known that you would be marching into a restaurant only to receive a coupon for folding chairs, you probably wouldn't be too enthused about spending your money at said restaurant to begin with. Similarly, most people would be discouraged at the thought of spending real money for a fairly high chance of getting fake money that you can't use to buy the thing that you want, so it would be safe to say that (relative to Overwatch's audience) a very, very minute amount of people were willing to spend a lot of money on the event.
Overwatch's Halloween event is essentially the same, but the fact that you can use the in-game currency to buy anything that you want may actually contribute to more people buying Loot Boxes. This time, instead of going to a restaurant and getting a coupon for something you don't want, you are getting a refund that would give you the opportunity to buy something that you do want. This gives off the perception that even if you are spending real money on the event, at least you are making real, tangible progress towards something that you want regardless of what the Loot Box is giving you, thus making it seem less risky than the Summer Games Loot Boxes. This probably isn't going to sway those who are adamant in their refusal to buy any microtransaction, but it would definitely have an impact on those who are less strict on the issue; you're receiving an item or progress towards an item that you want, plus you're (supposedly) funding free content for a game that you probably like, so why wouldn't you spend an extra buck here and there?
That's not to say that all of Blizzard's actions are driven entirely by the almighty dollar, but to say that the Halloween event is completely free of charge would be naive. Even if you personally aren't buying anything, Blizzard is definitely encouraging you to buy some Loot Boxes, much in the same manner that you aren't technically buying anything whenever you see an ad on YouTube, and if this event proves to be incredibly profitable (which it probably would), it would likely be prudent to recognize the reasoning behind any similar acts of generosity in the future.