If Product A and Product B are identical in every meaningful way, and Product A is unavailable, is there any reason the government should make it illegal to buy B?

No, but that is government. The Biden administration first created an infant formula shortage by overreacting to contamination in a small batch. Then the president said he didn't know his actions would create a shortage. Then after he was reminded the companies who make formula told him his actions would create a shortage he replied that they knew, but he didn't.

Meanwhile, a solution was always available; infant formula from Europe. The barrier there was that FDA-approved formula had giant tariffs slapped onto it so it wasn't sold here. Products identical to American versions, but made by companies that were unwilling to go through 10 years and $1 billion of FDA hoop jumping for the privilege of being forced by tariffs sell at a 15% higher cost than domestic brands, were illegal. Even though they are completely safe.

After an exhaustive look at the fiasco, the Biden administration has determined that the solution to the problem is more...government.

FDA is recommending that they get more money, more people, and even more power. Their report makes it sound like they are baffled by Cronobacter, despite the fact that the science community outside government understands it quite well. Only money and people will help there, they write. And they seem to think the problem really rests with the infant formula industry, for having one issue every 70 years. Only more FDA power over everything will fix that, they suggest.

Except it is a solution to a problem we don't have. FDA cannot and will not crack down on the fraudulent and dangerous companies the Clinton administration exempted from real oversight in 1994 and that profiteered from the infant formula shortage government created. They will instead crack down on the legitimate company that plays by the rules. And they won't allow, in an emergency, perfectly safe products to enter the marketplace without paying homage to an FDA process that has been an ineffective company buster for over 20 years.