People in here have been doing that for years.
It's 99% not to discredit.
It's for (a) making jokes, or for a laugh, (b) changing a word to offer a different viewpoint, but along the same lines as the original statement, (c) saying the exact opposite of what someone said, with multiple choice of reasons behind that, (d) and yes, sometimes to discredit.
But if someone says the exact opposite of what you are saying... that is part of the conversation.
You are the one overreacting as if you are mortally wounded by the tactic, rather than continuing the conversation.
Even if it annoys you because it is the exact opposite of what you said.
For example, I made a post that was NOT an "FTFY", but a straight up post that took your list of "Trump" positives and gave them back in almost the same point-by-point, in a post, with exact opposite viewpoints about how negative in each aspect Trump was. But it doesn't matter what the format of the post is (a "FTFY" that says the opposite, or a straight up post that states the opposite...); it's the content/ intent/ conversation of the post that matters.
You seem to be extremely sensitive to a re-wording of your post through a "FTFY"... If I may recommend... Don't be so sensitive about it... but rather understand the content of the conversation and respond to that aspect of the conversation that you deem relevant. Ask esweig... a fellow conservative, how he has responded to the same. He's been whacked with quite a few FTFY's as well. But we all have been. Some with an interesting alteration. Some for a joke. Some with an opposite viewpoint.
As for ewsieg: 99% he doesn't respond with indignity but, rather: "I know you think this way but I cannot", or "let's agree to disagree", or "funny, but I cannot stomach...".