I can see two possible argumentations:
After a headline, there is always a new paragraph starting. Because this is implicit, you don't have to additionally indent the first paragraph. However, a list does not necessarily denote the start of a new paragraph. Thus, if (and only if) a new paragraph starts right after the list, it has to be indented.
After a headline, there is some vertical space. This means the first paragraph is already separated from the previous text. It is an error to use an additional indent. Therefore, if a list is separated from its surrounding text with a vertical space, the next paragraph must not be indented.
Are there typographic rules for this special case? Is this a matter of personal taste?
LaTeX does put a paragraph indent after lists, but not after headlines. But it looks odd to me, and I typically suppress it manually.