Boeing doesn't make many of the parts in the aircraft, especially things like pressurization controllers. Those come from contractors like Honeywell.
What they do is design the systems around the parts, including selecting the desired level of redundancy, and commission the custom parts needed.
The 737 is still mostly a 1960s design built mostly to 1960s rules. There have been plenty of improvements but that's not the same as a clean sheet design built to be entirely automatic even when stuff breaks.
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.