"You have agreed to give some of your precious time away to your opponent to use as they see fit." (DKof)
No. Me and the other player, both of us agree that we are going to spend our time playing a game together.
You are mistaken. Time controls are very specific as are the rules. During your opponents turn, you have limited options. I hate to be the one to break this to you, demanding that your opponent has to move at a particular time or ever is not one of the options. Get use to it. Once your game has started, the time controls are set. No, sorry, changing the controls after the game has started would not be a "Good" thing. I would also caution against reading too much into someone agreeing to play a game with you. They might not have EXACTLY the same motives as you. They may be checking their CPU loading and wanted to open another (heavily scripted) browser window.
"it's only a game, what's so serious about it ?" (chrisr2212)
Games are mean to be fun, it is not fun to be siting there looking a timer count to zero. You should be more considered to other people, if you are no going to play more the game, then resign. What you win wasting the time of the other person?
"You have agreed to give some of your precious time away to your opponent to use as they see fit." (DKof)
No. Me and the other player, both of us agree that we are going to spend our time playing a game together. I totally understand that something important happens and the other player can't keep on playing, in that case, why no resigning before standing up?
" If they decide to spend their entire allotment on one move, so be it."
Sure, why no? people with not experience on fast live chess does this. I don't have a problem with this. My problem is with people who is not longer playing. People who is not thinking a move or nothing and they just let the other person waiting for something that is not going to happend.
"If you don't like the way they are using their time, resign."
One more time, they are using nothing, they stand up and go to do something else, this is not "using their time" this is wasting other peoples time. And they should resign before wasting the time of other player.