Restricting DLL injection and 'API interfacing' (if that's even a thing) to administrative users would provide no security benefit. People would just have to run as admin. DLL injection is used all over in windows to provide useful features like alternate input methods, icon overlays in explorer, etc. You aren't allowed to inject DLLs into processes owned by other users or processes of higher integrity level, so it's not really a significant security threat under the current windows model. IIRC Chrome and IE both run their processes at low integrity so those processes won't even be able to inject DLLs into other processes unless those other processes are also low integrity.