Humble's old model honestly never made sense to me. How could they possibly allow customers to allocate all their money to charities? How do the devs and platform make anything? It makes sense for them to impose this limit, but at the same time the limit's way too harsh. They should make it more like 70% max to charities, leaving 30% to divide between devs and storefront. 15% is just paltry.
They did have the standalone shop for quite a while too (late 2013 it launched, and they had widgets before then I think) which would be a more reliable source, possibly then making the bundles almost most advertising.
As far as making sense. I don't know if they were burning venture capital money (it was a spinoff from another company) or actually made something -- strictly speaking operating a casino is chance, however for all practical purposes then no it really is not. If it was the latter then they probably had very good data on how many people did what and what the breakdowns were as well as cost to them. If the devs as above mostly stuffed in games that had run their course* (
https://en.everybodywiki.com/List_of_Humble_Bundles , see list of dates of bundle to initial game release date as well as general lack of anything that is more service than game or multiplayer toy which is much the same thing -- think your minecrafts, terrarias, stardew valley, COD, battlefield, elder scrolls that was not the online that had already failed hard...) or maybe had a new game to advertise then their greater concern was probably cost of support (
https://www.puppygames.net/blog/?p=1574 ).
Now I would not be surprised if it was some kind of unfunded liability type deal, or trying to show a consistent income stream for the creditors (if they did one for PETA one month and everybody turned it off then the creditors might only give say 5x credit to some kind of metric, if they can have their charity effort go to grind orphans into stew to feed kitten fighting pits concern and still get 15% then that might be 20x credit to annual income or whatever) that caused them to do this particular course of action but the initial setup probably did make some level of sense.
*the sorts of things that 10 years earlier when things were still all on CD/DVD would be in a less than typical games retailer in spinning racks under expolsiv, sold out software, best of, or whatever your local banner of choice for that sort of thing was, 10 years earlier still maybe pure silver top CDs in a cardboard box proclaiming 5 flight sim/racing game/action game/... in one.