Because some people who read this manga are idiots who don't pay attention and like jumping to conclusions. It was obvious Kidd wasn't talking about Sabo, but the people who thought he was didn't see how Luffy and Robin reacted to what Kidd said.

Robin had a question mark in her speech bubble and Luffy wanted Kidd to tell him who this person with a "burn mark" is. It was obvious what Oda was trying to do, but some people chose to jump to the conclusion it was Sabo despite there being little evidence for it. Especially when there're tons of characters in One Piece with scars and burn marks.

Stupid people jumping to conclusions and throwing out random theories like that is commonplace in most fandoms. It's unfortunate, but you're going to have to learn how to get used to it. This isn't the first time people have done it, and it won't be the last.