Considering how comparitively -rare- mass murders are considering the number of "assault weapons" in circulation, punishing responsible gun owners for the actions of a few bad eggs is both unethical at its core and an abrogation of constitutionally guaranteed liberties. Especially considering that when establishing limits to any liberty, one has to ask whether or not the social benefit is worth the precedent the limitation sets; in this case, it isn't, because banning assault rifles would in all likelyhood not prevent such events from happening.
Why do they need a Hummer when a sedan provides the same functionality with vastly less risk in terms of accidents and ecological damage? The answer is; it does not matter.
Never mind that banning high capacity magazines is a stupid notion given how easy it is to modify magazines to increase their capacity. That, and it does nothing to reduce the number of said magazines already in circulation.