Because the desktop Laserfiche Administration Console is a Microsoft Management Console (MMC) application that uses pre-built MMC/Windows UI components for Active Directory and LDAP (LFDS) user/group selection queries. The MMC components allow multi-selection. The same Windows trustee lookup/selector component is used all across Windows. I'm guessing if it were easy to modify them to only allow a single selection, we would have done that instead of throwing the validation error in my screenshot.

If the LFDS (LDAP) query dialog dropped all selected groups as a single string into the LFDS Account field like your second screenshot shows without throwing a validation error similar to the Windows one, I'd potentially consider that a minor bug. But maybe the LDAP query component doesn't make it as easy to tell when multiple results are being returned vs the Windows one so the same validation check isn't there.

The backend code logic in that interface only handles registering a single user or group at a time.

Notably, the Laserfiche Web Client "Management" Users/Group UI does not allow multi-selection in the first place.

If you have multiple environments are and looking to reduce the burden of registering all these groups multiple times, you can do it once and then export some or all of the group list as an XML file you can import elsewhere.