Certainly it's hubris to believe that one can exercise "free will" to remain completely uninfluenced by one's environment. But what we can do is exercise a measure of intelligence and free will to anticipate the relationship between stimulus and response, and alter our environment accordingly.
I.e. if you're trying to lose weight, don't allow yourself to be exposed to fast food advertising.
Our brains may be heavily influenced by our environment in ways that we are not normally aware, but the existence of studies like these shows that we can begin to understand the dynamic of influence. Once we have understanding we can take appropriate measures.
As I see it, that's the true power of intelligence and human agency.
After all, as you point out, animals also have certain powers of inhibition. But none have the ability to comprehend their own minds and alter their environment accordingly.
"Unless we manipulate our surroundings,we have as little control over what & whom we think about as we do over the muscles of our hearts."
Think of a lion in the brush, ready to pounce on an unsuspecting antelope. The lion wants to pounce, but inhibits the response for a little while, waiting for the antelope to come a little closer.
Since willpower is depleted with use (a process called Ego Depletion), inhibition is a very limited power - it's difficult, requires a lot of energy, and can't be maintained indefinitely. Our natural inclinations and environment are extremely powerful forces, and we can't hold out for long.
That's why, if you're experiencing some form of temptation (wanting to do what you know you shouldn't), it's better to inhibit and remove yourself from the situation as quickly as possible vs. rely on your "free will" to save you. Otherwise, you'll progressively find it more and more difficult to resist until you act.
I find this definition very useful because it maintains human agency (the ability to choose your behavior), while respecting the undeniable power that environment and circumstances have on our behavior.