Or people having just a mild variant which does not cause fever and airway problems. If they think it is something else, or don't care, it is just as well a transfer risk as asymptomatic individuals and in my opinion at least as likely.
If you have the snuffles, are you supposed to react like it's COVID, or react like it's just one of the million other colds and such going around every winter until things get bad enough that your symptoms concern you?
If you react like it's COVID, then what - go to a hospital or otherwise burden the health system? All they can tell you is to quarantine yourself, unless/until it gets bad enough that it's worth testing you.
Another one is risk factor, like any ties with people who are infected or people who came from high risk areas.
If you just have the sniffles you probably shouldn't panic and go to the ER, but you should stay the hell away from big conventions and not go to work, at least within reason.
I work for a company that has unmetered sick leave (that you can take without people judging you), great VPN/work from home policy, and will actively shun people who come to work sick (that is, no one is even remotely close to being pressured to come for butt in seat time).
And people still come to work sniffling and coughing. I keep having to kick them back home day after day. Like, wtf, seriously.
Actually, as I'm typing this Im hearing someone coughing and sneezing a few rows from me, and yet again Ill have to talk to them and maybe even their manager, because here we're super privileged and there's exactly zero reason to come in sick.
I agree not everyone has that privilege, but if those who DO actually exercised it, it would at least have SOME impact on the spread.