It's a violation of the COC to book a ticket without the intent to fly it. So, how does UA determine intent to fly? That's the question.
I would surmise and it's just surmise, that UA cares a whole lot less about cancellations within 24 hours for a time down the road than the GS who had his MP account closed because he cancelled refundable F tickets at the gate.
The same for "impossible" reservations (multiple fare locks). At least they called.
It's all about inventory spoilage. If this starts to affect the bottom line, you can bet UA will care a whole lot.