I would politely disagree, it's entirely about one of the most basic issues in the constitution. It's not about a "right to buy" it's about the right to bear.

There is a specific constitutional amendment regarding the rights of the people to bear arms, some would argue that that right would include relatively unfettered access to ammunition.

There is no constitutional provision or amendment that addresses the peoples right to buy specific medications, fireworks, automobiles, homes, groceries, toys, etc. I would submit that if there was a constitutional amendment guaranteeing the people the right to own/buy pseudoephidrine and a state took it upon itself to aggressively moderate the people's ability to do so as this bill does with ammunition, that would then become a constitutional issue.

If a store is having it's inventory stolen, shouldn't the steps taken to alleviate that be their responsibility, not the government's?

There is nothing in the written versions of this bill that addresses shoplifting that I've seen. There is nothing in it that addresses the issue of stolen ammunition being a problem in the first place. One might assume that if having ammunition stolen were such a problem that the retailers would take the steps they felt necessary to try and stop it themselves. The fact that a lot of stores have ammunition sitting on shelves accessible to the public leads me to think that having it stolen isn't a major concern. If the bill in question was about helping to stop the theft of ammunition, why then is it limited to ammunition used in pistols? Seems to me that if a person was of a mind to steal ammunition in the first place, they'd steal whatever they can get their hands on. I'd submit that using the theft of ammunition as a reason why this bill is important is an attempt to throw sand in the bulls eyes, as I think Plato said of rhetoric.