Discalimer: By the "you" I use in this post, I am not referring to you or anyone else in particular. Also this is not a rant, just a straight explanation.
No, it is not annoying, it is a set rule and a certain amount of time is equally available to everyone. NON-extendable time, right? They don't use your decision time, they use theirs. Some use it as a tool to annoy you, I mean, why not use it if it works?
It is not a personal matter.
Let's make it as simple as possible:
Everyone has an alotted time to do with it whatever they please, as they please. Why does it matter for you what they do with it? And the simplest answer is: because you are an arrogant child and you want something that you are not getting. You want everyone to play for you, like you and to your advantage and pleasure. No one should care about you in a game meant to take your money. But there are set, fair rules for a reason (you) and thankfully you can't do anything about it.
It doesn't matter if you play 1 or 100 tables at the same time, if you scubadive, cook, jog, have sex, bathe, swim or whatever else you do in the comfort of your environment while you play online poker. Everyone still has the exact same allotted time and they can use it to make a decision (simple or difficult) or to help themselves get in the money or to annoy you (which means you are weak, not them, so it's your disadvantage) or to juggle another activity along with poker.
Even simpler: Daniel Negreanu, publicly and freely available on youtube, plays live poker at a live table with a tablet in his hand where he plays an online tournament at the same time, while he films himself doing it. I'm sure he takes his time with every decision he makes.
And don't forget to have a laugh about it!
Hope this explains it. It's really simple.