There are arguments aplenty that schooling and a minimum amount of healthcare are public goods, as are roads built on public land (the government owns most roads after all).

What is the justification for considering data centers capable of running LLMs to be a public good?

There are many counter examples of things many people use but are still private. Clothing stores, restaurants and grocery stores, farms, home appliance factories, cell phone factories, laundromats and more.