Hi,
Yes, you are right, this is not controlled by WEBCUIF. That is it is not in CRM scope or Web UI scope. It is more BC control.
I don't know if it could help to realize your requirement, but there is control to skip this screen:
- transaction sicf, service crm_ui_start
-> "Error Pages" tab
-> "Logon Errors" tab (inside "Error Pages")
-> "System Logon" should be checked
-> Click in the "Configuration" button
-> In the section "Actions During Logon",
"Check for Multiple Logon"
I guess in your system, the above flag is activated. If you deselect this flag, the page won't be shown. But the multiple logon will still be possible. 
The number of the logon can be limited by icm/HTTP/esid_max_ctx parameter.
Hongyan