Ability to switch to another OEM OS

Dear Dell support,

I've spent a lot of time on-line reading about inability to move between OSes on the laptops which have official support for both, Windows and Linux.

To be specific, I am currently in the process of purchasing G5 15 5500 with Win10pro. I want to use Ubuntu on it but it seems that Dell does not allow the access to its Ubuntu 18.04 installation recovery media, unless the laptop was purchased with Ubuntu specifically
Could someone please confirm this officially and provide the reasoning behind it, if it's true?

Thank you very much in advance.
--
Dejan