Handcuffs and weed? Seems marijuana is not legal in all states... in the "United" States of America

In the above photo, you can see that the handcuffs are cuffed. Meaning marijuana will get you cuffed (arrested)  if you're in possession of it.

In the photo below, the handcuffs are uncuffed. This is because in different states, marijuana is legal and you are allowed be in possession of it and you won't get arrested for it.

Confusing right?

So, the question remains: Why is marijuana only legal in some states? If this country is called the "United" States of America, why is it that the laws are so different from state to state? Shouldn't all states, if "united", have the same laws, especially if it is legal in states for medicinal purposes? Medicine is medicine. As long as one has a prescription for certain medications, it can travel across state lines. But, not marijuana. Surely there is an issue at hand.