Well it's matter of comparing it to their competitors. GOG actively advertises and encourages devs to eschew DRM, lots of games became DRM-free just to appear on the store. Humble also actively advertised DRM-free games, and even source-available games, though granted, nowadays it's more of alternative steam store interface with charity, with very few direct download titles. itch.io, due to the area they operate in -- niche indies -- it's pretty much cultural standard to have no DRM, so they don't even have to mention it.