Humble Bundle announced back in April they would be making changes to the purchase sliders based on (much hated) testing, then backpedaling in May, they've announced again some changes are coming to always give Humble a cut.
Previously when buying a bundle you would be able to adjust the sliders of who gets a cut, to give 100% to charity if you wish (or developers/partners) and you could even give nothing to Humble. That's going away. The sliders themselves this time will continue to be an option, so you will be able to adjust who gets what. However, Humble have made it clear they will always have a minimum cut between "15 – 30%". This brings them more inline with other stores but it does mean it's far less generous to charity and developers.
This change comes after 10 years of Humble allowing people to give them nothing. To play "devil's advocate" for a moment: it's not even remotely surprising they want to do this though, they are a business and they do obviously want to always be able to pull in money from customers, they aren't a charity themselves. However, after enabling full customization for this long, it's obviously still not going to go down well with a large portion of their customer base.
Explaining why they're doing it now, Humble said "The PC storefront landscape has changed significantly since we first launched bundles in 2010, and we have to continue to evolve with it to stay on mission. The update will allow us to continue to offer great prices on amazing games, books and software all while supporting important charitable initiatives with every single purchase" and that the change will allow them to invest more in their content. They also noted they're closing in on $200 million in donations to various charities.
In an email with Kotaku, Humble boss Alan Patmore said "Operational costs, such as the cost of acquiring content, have risen dramatically" which is one of the reasons why they're doing this.