For every person willing to spend time and money converting an AR-15 to fire fully automatic and then rob a bank in Los Angeles and THEN have a 3000 round shootout with police to the death, there are 10,000 other people unwilling to do so.And it will just increase the rate at which they are stolen, and cause more violence here in the US.
You are also missing the point that the law did not deter them from committing the crime. Laws do not deter those who wish to break them.
Laws deter many people. You cannot use the exception to argue against the rule.