How do you plan on taxing something that is free?

I think that you're still one level more abstract than what is happening here.

Tariffs are set for classes/types of goods/materials/etc... and Kinder Eggs were banned because they didn't comply to a regulation (small dangerous foreign objects contained in the product - maybe other similar products had to be banned as well but those ones didn't reach the headlines).

In this case the decision applies to a very specific company/product and that decision is not based on a rule/regulation/etc..., I think => on one hand it doesn't have any impact on other similar companies/products, on the other hand it scares me that a single person is able to enforce on-the-fly these kind of decisions.

Yes, it's similar to tariffs. Tariffs are a bad idea, and so is this ban.

But should it?

Such naivety on display here, I am alarmed for HN. National security isn't blanket grounds for doing anything. There are limits to president's and executives and even legislatures authority. Arbitrarily nothing can be done in our system. Unless US government probably demonstrates to the public that this measure is required, such a ban cannot be condoned.

They can do just anything, once they convinced you it's "because national security". And it seems that people are very easily convinced.