The French government isn't dictating how it's citizens spend their money. It's dictating how Amazon can sell things to its customers in France.

Because competition is good in theory but, in practice, many kinds of competition have side effects. Many big companies (Walmart comes to mind) have the capacity to distort the market they're in, change the rule of the market itself. Those external effects that goes beyond "simple" competition may be considered unfair.

For example in France, Carrefour ( the french walmart ) used dumping for a few decades, destroying any competition, then slowly increased its prices. Logically, competition should emerge but Carrefour is so entrenched into the infrastructure of the market that a competitor cannot really survive. This is a simplification (the french market is more an oligopoly than a monopoly) but the overall scheme is true.

Amazon may or may not have one of these effects. I don't (yet) know or more precisely I don't have enough info to have an opinion.

because the State pays for long terms problems, always?

Are you arguing against the existence of economic and commerce policies?