Currently Fletcher defaults to rendering the contents of a !math command surrounded by dollar signs, for inline-style math. However, given that the math image is the only thing in the comment, display-style $$ probably works better as a default.
The one advantage I can think of for making this the default is that it allows me to use either style easily: !math expr renders expr in inline math stye, whereas !math $expr$ renders it in display math style. That wouldn't work if display style were the default because $$$ doesn't mean anything to TeX, but it's unintuitive enough as a shortcut that it probably isn't that helpful anyway.
(Ideally it would default to display style but have some way of easily switching to inline hardcoded, but that's higher-effort)
This has been done, since using
!latexinstead of!mathdoesn't put$around the text at all, and if you really wanted inline behavior you could!latex $math$.