You know, I don't disagree with your basic point, it's right on in fact.
However, applying it here is massively overstating the case. These guys don't store ammo for you for any appreciable length of time unless that's what you want. In most cases. I'd gather, the shipping threshold is short enough for that not to be a significant issue.
Overall though I tend to agree with you that I'd rather just buy it and keep it in my possession. I just thought this was kind of a neat idea.