Please explain this drug pricing

I don't understand this, do you?

Here's a full page ad from Pfizer in the New York Times that, it seems to me, makes a prima facie case that the company is engaged in price discrimination against (1) the federal government through Medicare and ACA Exchange plans and (2) uninsured people.  If you are in those categories, you pay $30.  On the other hand, if you are with a commercial insurance company with one kind of policy you pay $4, and with another kind of policy, $30.

I guess this is legal, but it sure makes you wonder how it can be, or whether it should be.