Which sign do you notice first? Probably the one on the left, despite the fact that both signs say the same thing. The improvised nature of the one on the left calls attention to it. Its freshness implies a willingness to act and investment in enforcing the rules; it has been deliberately added to a pre-existing built environment and is therefore a reflection of the opinions/priorities/thoughts of those who use/control that environment, not those who built it. It’s a sign of recent human activity, essentially, and just like you’d be careful if you saw fresh bear tracks, you know to be careful about breaking the rules when you see fresh human tracks.
For the record, the four seats that Amtrak has marked as “reserved for passengers with disabilities” were occupied by two moms, each with her elementary-school-aged daughter.