Feed readers can send If-Modified-Since or If-None-Match as part of the request so the server only sends back the full feed if there's something new (Otherwise a 304 Not Modified)
Assuming that the feed implementer bothered... many news CMSs don't bother. The world of RSS is a horrorshow of standards noncompliance; Atom only somewhat better. (My experience is, admittedly,a couple of years out of date, but feed generation tends to be a minimum spend item for news organisations.)