Prusa gave everyone permission to make copies of their i3 series machines, make modifications, and distribute modified versions. It’s not theft if you have been given explicit permission to make copies.
“You may copy and distribute verbatim copies of the Program's source code as you receive it, in any medium, provided that you conspicuously and appropriately publish on each copy an appropriate copyright notice and disclaimer of warranty.”
“You may modify your copy or copies of the Program or any portion of it, thus forming a work based on the Program, and copy and distribute such modifications or work under the terms of Section 1 above.”
https://github.com/prusa3d/Original-Prusa-i3/blob/MK3S/LICEN...