The lock screen and display manager are two separate pieces of software. The display manager runs before a user profile has been selected for graphical login, so it does not have access to your user desktop/display settings

Free software licenses (e.g. GPL) allow repackaging and selling the works, they just also require that any of the ‘repackaging’ is also under a free software license. You can even sell someone a piece of free software without making any changes, nobody is going to buy it though because it can usually be found for free somewhere else.