The people getting upset are the objects of the hate. There are plenty of echo-chambers to play in, things like this are on Twitter for a reason.
Firstly, overall, a lot of the aim of this sort of posting is to make one's own side upset. The alternative to making the other "upset" is to promote alarm, fear, anger in one's own, or incite derision and contempt from them towards the other. Although individual items might sway one way or the other, overall they are different sides of the same coin, and I doubt you would find many who employ one and not the other. The generation of emotion to attract attention and carry a message thus works to both rally friends and fray the enemy.
Secondly, echo chambers are inherently restricted, with limited audience. If you want to be noticed, or to get a message out, you are infinitely better on X (fka Twitter) than any echo chamber. There are other reasons, obviously - someone may be on Twitter for other reasons, and cut loose on there just because it's the space that they happen to hang out.