Possibly because that bus system has no fare, or they were having an exasperating day, or you are so bad at paying a fare that its easier just to have you sit down?
Happened to me before. The ATM only gives out 20s but the driver only wants small coins, only cash is accepted.
It happened in several countries and it's still a mystery to me how you're supposed to get small change when you're fresh off the airplane or something (or why they don't just take card).
London for example - you don’t pay the bus driver. Theres terminals on the bus to scan your payment card. Distracting the bus driver when there is no need would be annoying
If we knew what city/route/service and day, we might be able to get a better idea.
Sometimes operators declare a 'fare holiday' when everyone rides free, usually as compensation for some major fuckup previously, or for some other PR stunt. Metlink in Wellington doesn't charge on Christmas Eve, Christmas Day, or New Year's Eve.
Operators sometimes half-strike and refuse to collect fares.
The specific route, service, or time of day might be free.
It's an express service that you can't pay cash on (only fare cards) and it's easier/nicer to tell you to ride for free than to tell you to get the next bus because they don't take cash.
You might be part of some group (youth, students, elderly) that doesn't have to pay.
Something is broken and they can't collect fares.
They don't want to deal with the big banknote you had.
Most of the time, the fare thru collect is nowhere near enough to pay for the service they offer. However, partially because is capitalism, they can't give the service to for free because, well, reasons.
The fare is useful to track how many ride different lines at different times of day stuff like that. Plus, it gives a way to count how many people use the service for the purpose of getting grants and whatnot - which is what actually funds the bus service.
So, if the driver is in a hurry for any reason, he'll often give you a free ride...cause actually paying the fare is, well, kinda for appearances, really...