IMHE, most guys (NOT all) bottoms or not want to get their rocks off, too. So, they shoot their loads at some point during the play.
I am also sure that most tops find this to be somehow satisfying, confirming their ability to bring their bottom dudes to climax. A few folks find that climaxing together is a very valuable bonding experience confirming that the sex in sync may be indicative of a few other things that are also in sync, leading both dudes to believe that whatever kind of relationship they may be pursuing is actually on solid footing.
Now, whereas all of the above may be your daily default, even an inexperienced top is likely to understand that his bottom may not want/be able to shoot his load during the play. I doubt that too many top dudes would see this as a problem of any kind whatsoever if clued in beforehand. If you choose to bttm, you are perfectly free to provide a hard one spurting ropes of jizz during the play, but you do not have to. You are bottoming, so the focus of the action is really not on your cock. That's why you hooked up with your top dude.