If an act is only a crime when used to prepare for a more serious crime, then the crime should be 'preparing <serious crime>' not the preparatory act. If an act can be shown to be preparation for a more serious act then the person should be punished for preparing the more serious crime (which usually has a maximal punishment equal to actually committing the crime), if there isn't enough evidence to show that the preparatory acts shouldn't be punishable either.

It really seems like a way to try to convict someone with shoddy evidence, or some roundabout way of trying to get around double jeopardy, or perhaps to criminalize intent, rather than acts.