I think it is a simpler case of legacy. They had the Linux/Windows/Mac symbols before proton was a thing and back then you needed it to be a native Linux game for it to work on Linux. Or you had to install all of steam inside wine and had pot luck as to if anything would work. Since they released proton they have kept the OS symbols the same. And since they released the steam deck they have added new deck verified/deck playable symbols which are a much better indication of playability on Linux.

Maybe not the best marketing. But I don't think it is really due to legality issues, more so legacy ones.