Remember, Scripture says satan was perfect in all his ways till iniquity was found in him. What was his first iniquity? I believe it was PRIDE AND UNHOLY AMBITION. He was proud of his power and beauty, forgetting it was GOD who'd bestowed them. He aspired to be like the Most High and sit on His throne. Ever since then, he has continually tried to PROVE GOD WRONG, as he tried with Job.
And, yes, God gave all of us, angel and man alike, the ability to MAKE CHOICES. However, he is much more strict with the angels than He is with us because the angels see Him directly every day, and are without excuse for rebelling against Him. Thus, the angels who sinned are confined in tartarus till judgment, with no chance for salvation as we have. Thus, we should thank Him every day for what He's done for us!
And I completely agree with the robots statement. If God had wanted robots, incapable of disobedience, He woulda made them. I believe He wants beings who love and obey Him of their own free will.
Why did God allow Satan to go on sinning and lead others to sin? Because His JUSTICE is perfect. Also, He has appointed a time for the age to end, and until then, He goes on creating people to serve Him if they will. When Satan is consigned to gehenna, every intelligent being in the universe will know why. Everyone who dies in sin & is consigned to gehenna will know exactly why. All will be rewarded or punished according to their actions prior to judgment time.