*sigh*

It's not!

However, hiding a cellphone -- for instance, one rigged to serve as a component in an IED -- under the lining or inside the hollowed-out sole of a shoe *would* be considered artful concealment ... and most of the time, it's not going to be possible to determine from the X-ray image whether the item is simply lying in the shoe, or concealed within it.

BTW, Richard Reid isn't the only person to have attempted to smuggle a weapon in a shoe! I believe a female passenger managed to hijack a small coomercial plane in New Zealand a year or so ago wielding a knife she had hidden in her shoe. She managed to slash up the pilot, too! (Sadly, the passengers did not intervene and disarm her.)