Game will be windows first and foremost. Then ported to Mac and Linux. So extra work is being done on it Hence this became reality once it hit 1.25 (or thereabouts). I don't see how people can justify asking for extra copies outside whats offered for each tier. All that will do it reduce the copies people will want to pay for.
There's a post by a Rachel Teng that best sums up reasons against offering multiple copies for one payment just cause you use multiple OS's
There is no extra work involved once those other copies are created and the Steam version is apparently going to cover both Mac and Windows, leaving the DRM free version as a single platform.
Once they do the work to create a Linux version there is precisely zero additional cost to them and expecting to be paid multiple times for the same person to play is just silly. Most other games I've bought provide all OSes that they support for the same price because quite frankly it's unlikely that anybody is going to want to play through both versions at the same time.
The only reason people want both is because they're likely to not be committed to going Linux permanently or think they're likely to stop using Windows at sometime in the future. What's more the Linux version is essentially future proofed as you're almost certainly going to be able to install it later on, or maintain a separate install with just important things updated.
If they want extra money for the licenses fine, but making people pay double for something that's definitely not twice as good is just getting greedy. I don't think there's anything wrong with wanting double the money for the second license, but requiring individuals to have two licenses in cases where they're dual booting, but not when their using the same version on multiple installs just makes no sense at all.