>if some joker wires a 15amp plug to a 30amp device
Or plugs two 10 amp devices into one duplex recept.

>why cant I string 6 15amp duplex recp. on a wall with #10 stranded in the emt, than just pigtail or "tap" the recp. with #14 solid. slap a 30amp breaker on it and whamo. Can I do this?
You may not go over 20 A on a general purpose circuit of that variety.