If I recall, cancellation penalties were one of the 'new deals' from Anderson, when he took the reigns a few years ago. It was 'easy money', just like the airlines do.
As Zephyr17 noted, back in the pre-online booking days, one would call 1-800-USA-RAIL number and make reservations. They'd take your credit card number and mail the tickets, if desired. Or, you could trek to your friendly nearby Amtrak station agent and get the tickets there. I think a hold of 7 days was the maximum before your reservation got cancelled.
Part of the reason for the short 'hold' was the extremely limited number of bedrooms and roomettes on a train. Two 10-6 Heritage fleet sleepers were 32 rooms. Two Viewliner Is are only 30 rooms. Contrast that to a cruise ship that has over 1000 rooms (so I'm told). If they have a cancellation a week before sailing, they'll not likely sell that room. So, at worst, they've lost 0.1% of their revenue for that cruise. Lose 1 room of revenue out of 30, you've lost 3-5% of the sleeper revenue (depending on accomodation) for that train.
Note, too, that from what I've read here and there, at least for NEC trains, most of the sales are within 2 weeks or so of travel. Yes, many vacationers will book their rooms as far in advance as possible. But in 'sort of' watching the fares jump around for sleepers, pre-covid, at 5-6 weeks out was when they'd sometimes drop the price to sell more rooms. To me, that indicated they hadn't sold 'enough' according to their pricing algorithms. And pre-covid, at least, there was frequently more than a few that would make their reservations less than 14 days in advance.
With 50 years history under their belt, and especially the last 10-15 years with more detailed data (massive 'big data' data bases, etc) and improved statistical analysis methodologies, I'd guess they have it down to almost scientific levels figuring the probabilities of room sales 1 week, 2 weeks, etc in advance of travel. Certainly there's been a couple of former Las Vegas 'odds makers' there to show them how to do it.