I recently cross-saved to PC and it tried to have me buy it again too. I just wanted to use pc to work my weekly stuff when my kid is on PS and I thought I was going to have to purchase the expansions as well. It kept offering for me to buy it. However, I declined and when I started playing on PC, I found that I already had access to all of it on PC without having to re-purchase it - possibly because I tend to purchase by clicking through the game on ps vs searching on ps - not certain... I'm not certain why it's doing this to you, but just wanted to let you know because I chose not to re-purchase the expansions and the game itself already had all the content available without me having to re-purchase from steam.

The issue is when you purchased shadowkeep you only purchased the license to play it on that platform. Its like if you bought Call of duty on Xbox, you only bought it on there so you would need to buy it again if you wanted to play on pc or playstation. You currently do not own the license to play it on pc despite having the license on Xbox. Please note this isnt bungie's fault and its just a typical issue with moving consoles and having to rebuy many games. Though something I would like to note is that the season pass that can be purchased for silver can transfer between platforms as its a micro-transaction purchaes and therefore has nothing to do with platform licensing.