If you are a student who teaches the login screen, there are three scenarios to consider.
The first two situations are common when a student had their account created with LTI.
Only the third situation is something where it makes sense and is security-wise to acknowledge the person is a student and actively redirect to the LMS.
How would this look in practice?
A user reaches the login screen. WeBWorK does not know yet if this would-be user will be an admin/professor, so the screen will show the same thing it shows for a student who has reached the login page. So what next?
There would need to be input fields for the username and password. Students will try to use those, thinking they can use their regular institution credentials. That will fail and they will be frustrated.
I can imagine some ways to mitigate the issues there, but ideas I've had are not great. Maybe you have a better idea though.
My not-very-fancy solution has been to include some text in the site info file along the lines of "Student login is not possible through this page, please access your homework via Moodle."
As long as instructors take a moment to ensure students know that they need to log in via LMS, there aren't usually many problems.