Fully agree. I think it's a poor move by Amtrak - if nothing else Amtrak's losing out on revenue from a bank buying points for cardholders that can't sign up for the card right now but would if they could.
I'd worry that any increase in credit card earnings would result in Amtrak actually devaluing their points even further. Right now points can be pretty easily redeemed for around 2-2.5 cents per point, and for sleepers are closer to the 2.8-2.89 cent range (the direct conversion is 2.89 cents in many cases, but there's some caveats to that, particularly in coach where discounts don't apply and saver fares aren't available for reward redemption, so I value it lower for coach redemptions.) Most airline miles, particularly for non-partner itineraries, are closer to the 1.2-1.4 cent mark, so roughly half the value.
I looked at MSP - SEA between Amtrak and Delta (both of who run direct routes.) While Amtrak has quite variable pricing (with a lot of sold out sleepers - add more sleepers Amtrak!) the cheapest I found for a roomette was around 19,000 points, with other dates at around 38,000 points and a couple as high as 50,000+ points. On Delta, the cheapest first class seat I could find was 42,000 points, and coach was still between 12,000-18,000 points even on relatively cheap itineraries. To be fair, the Delta Gold card ($99 AF) earns 2x on dining and groceries, where the AGR card with an annual fee only earns 1x. However, the AGR card (AF one) earns 2x on all travel, and 3x on Amtrak travel, one more point than general travel or Delta travel on the Delta card. Granted, you can transfer American Express points over to Delta where Amtrak has no similar partnership, but if you're going just on direct card earnings I'd say you'd get better value for spend with Amtrak with their BoA cards.