I can't say that it's definitely the case here, though it sounds like it, but some sites that offer subscriptions only send emails to subscribers deemed 'active' by regular patronage. In addition to it being an incentive to visit the site at least semi-regularly, it avoids wasting time and resources pushing content to people that don't seem to be doing anything with it.
In addition, some portion of people who initially subscribe end up changing their mind send it to spam, instead of unsubscribing etc. Having your updates reported as spam is never good, as it can get your domain entered into indexes as a known spammer and that makes it harder for the people that want to get updates also. So once again its in the sites best interest to avoid excessive transmissions to inactive users.
No matter what the threshold is set to, whether it's days, weeks, months, or years invariably there will be people that miss an update and think it's unreasonable for them to be required to log in that often.