Steel pieces don't get covered by one-way walls anyway once actually playing - they do in the editor, for much the same reason that objects do (ie: the editor's rendering code is not very good), but once you actually play the level, any pixel affected by a steel area will be non-one-way-ized.
The problem with this change is - while it may be slightly more intuitive for new users, it would be more annoying than anything else for experienced users. If there is enough support for this change by people who feel it's beneficial - not just on theoretical grounds, but based on their own actual use scenarios, be it from while learning or how they'd prefer it now - then I could look at adding a configuration option on how to set this property for terrain pieces, and setting the default to the new-user-friendlier setting.