[arp242]

They can walk away, but the government can levy additional tax if they do.

But realistically, from a moral point of view, they can't really walk away in the first place unless the government demands are ridiculously unreasonable (e.g. demanding a price lower than production). The only drugs subject to negotiation are those without generic alternatives (i.e. where a free market doesn't really exist), so stopping supply would quite literally kill people. This gives drug companies an enormous advantage in negotiations in the first place, so this kind of compensates for that.