The electrocution itself causes the muscles to tense and grip. Normally when you use your muscles, nerves send electrical signals as action potentials.

Same idea as lego. The objective is to do whatever you want. I personally have never found any interest in the game, but plenty of people do

Because the provider doesn't allow it in the contract? Their purpose is to make money, after all. It's a balancing act between how much to charge people vs how much stuff to make available for free, to farm goodwill and gain attention/accessibility