There are thousands of people out there who have done partial boils and use regular old tap water to top off the fermenter with, without boiling or otherwise sanitizing it first, and I'm sure that the vast majority of them have successfully brewed beer that way without infection.
The risk of adding several gallons of tap water to your fermenter has got to be a lot more significant than giving your equipment a quick rinse with tap water after sanitizing, and yet people seem to freak out a lot more about the latter. (Not trying to say either is a good idea)
But what's the excuse for using tap as top-off water? More often than not, I think it's that pre-boiling and cooling your tap water is a hassle, and buying bottled water costs money. It's not a GOOD excuse, but it's a common one. Experienced brewers warn against it, but people still skip it.
But rinsing after sanitizing with starsan is practically the opposite. It requires more effort, there is no benefit, and if anything it increases risk of infection (even if it is by some probably negligible amount). Experienced brewers warn against it, but people keep doing it.
I think the moral of the story is that sometimes people just don't want to listen to advice

Personally, I'm one of those who runs around with a spray bottle full of starsan and uses it on everything, and never rinses after sanitizing - I can't understand why anyone would expend MORE effort to be LESS sanitary - to each his own. But I also acknowledge that rinsing is not very likely to cause problems - again, to each his own.