- They do happen
- You're less likely to see such posts when teams are winning, which explains why you haven't seen it here as often as you might see it elsewhere.
That claim likely depends upon how you're defining "rigged". There's no question, for example, that assigning certain officiating crews to certain teams will have a tendency to make things easier/tougher for said teams.
Not really. The second is just a fatalistic outlook via a pessimistic view of the former.
I've never yet seen a single fan base, in either basketball or football, where people don't claim that game outcomes are predetermined, or at least attempted to be predetermined, by the league. Hockey and baseball are a bit different, and the number of people making such claims will vary according to team success and other factors (history of getting screwed, as with the Saints, for example). But, even there, people will talk about specific officials and their potential impact upon game results.
There are a few outlier fan bases in sports, but most fall within a small range (in the context of the particular sport/league), in terms of how they behave at games, publicly, and in social media. Chiefs fans tend run a bit more to the pessimistic side, because of their "recent" history, but that doesn't put them outside of any overall norm.