If you want to be a "Big Boy" country, you have to have your own monetary policy and currency. The problem is, managing your monetary policy and currency is really difficult. Countries that experience hyper-inflation are usually ones where monetary policy isn't done right. The other side of it is, when you do have the right monetary policy and currency controls, it allows to survive and possibly thrive in the financial world.
> The present currency of Ecuador is the United States dollar.
There are a number of countries who's currencies are pegged to the US dollar at fixed exchange rates. Notably Hong Kong, as well as gulf states such as the UAE, Saudi Arabia, Qatar, etc.
This means their central banks have very limited scope to control domestic money supply and interest rates via monetary policy.
So it kinda does make sense they accept it, since they can ultimately use it.