they do if they don't want a user's ISP to inject ads or modify content in transit.
this does happen
this does happen
If the law forbids ISPs to show ads or modify the content in any way, then they won't do it.
Why that statement, because any law to ban ISPs from doing this will be met with millions in political investment by said ISPs to prevent it. Since the average person doesn't care about this issue it's a losing proposition.