Because it is their decision to do so. Mojang could've done that too. In which case few of us on the forums would now be playing Minecraft. Funny how those mods which contain said DRM are getting steadily less and less popular, isn't it? Gee... I wonder if there's a correlation...Interesting that these mods exist for the very reason that Minecraft contains no real development-restrictive DRM.
Why is it that mod authors should be allowed to take advantage of such a cracked-open system, and yet also apply new restrictions to their own derivative works on a whim?
Of course, the whole argument is entirely irrelevant. Slowpoke does not have control over what Mojang or the mod authors do with their code. If they want to do that, then that is their decision. Slow is producing a Launcher and several Mod Packs. As long as they function without harm to the end-user, it isn't his ball of wax.
If you truly feel this way, appeal to said mod authors directly on their forums, or to Mojang.