StarCraft Remastered: should I buy it for every OS?

I’ve purchased SC: Remastered for macOS, but then i bought a PC, and wanted to play it on my new PC. But when i downloaded and installed StarCraft, I realized that it’s old StarCraft orinal, not remastered. And I was offered to purchase it again.

What is it? Should I buy it again, for Windows? Or what?