In my experience, it is normal for the default gateway to be assigned to the wired connection (even when there is no wired connection).
Yast → Network Devices → Network Settings
Select the wired interface, and click “Edit”.
Select the “General” tab. There should be a setting for “Activate Device”. It is probably set to “At boot time”. Change that. If you never use a wired connection, change it to “Never”. Otherwise, change it to “Cable connect”.
Then see if you get a default root for WiFi. Hmm, you might need to reboot (not sure on that).
The other alternative – perhaps even better, is to switch to using NetworkManager.