Just like you know what the person is and what is coming when someone says "I'm not a racist, but.."

[RB TexLa]

Same now with "I'm not a protectionist, but..."

[spoony]

1. Um...context?

[uppityperson]

2. I am pro-choice, but...

Can be used for a lot of things.

[elocs]

3. "But" usually means the first part of the statement will be negated or qualified

to one degree or another.

[Wiley50]

4. Sorry, but I don't buy that line of prejudice

Sometimes a person is just perceptive enough to see
that some would immediately see an opening and jump their ass
when they were actually misinterpreting where the speaker was going.
and so they preface it that way.

Sometimes it's like what you are alluding to
but sometimes it really isn't.

So your blanket "Gotcha" just doesn't wash

Sorry