The mATX made for general purpose by major Mobo brands are ATX case compatible.
mATX that are made for manufacturers use, like Dell, Gateway,HP, etc. are many times Not compatible with standard ATX cases.
mATX is designed for small compact cases, so putting it in a Regular ATX case is a good thing, cause there is much more space to dissipate heat.
mATX are inexpensive solution for computer builders that sell commuters to regular used that do not care about the "Rig" and whats in it as long as it does well regular chores, like Internet, email, and word processing. These type users are by far in greater numbers than enthusiasts.