Hi,
Virtuemart Core is responsible to put asterisk (*) next to them, but since those fields are not required for "Guest Checkout" (which is actually a different scenario) this is just a small bug that needs to be patched.
You can use this patch to fix this problem with ease.
Open this CSS file:
/templates/flexible_sportmart/css/custom.css
and insert these rules:
label.username:after,
label.name:after,
label.password:after,
label.password2:after {content:'*'}
and there will be asterisk (*) next to them.
Regards.