Mandated by the government?
Sure. If you buy a CB, it comes with 40 channels... Mandated frequencies, reserved for Citizen's Band radios. You can buy handhelds with one, three, or five channels of your choice, and the only mandated channel in all cases is channel 9, which is the emergency channel.
Likewise radios in automobiles - AM is reserved frequencies, licensed for use by broadcasting stations. If you supply a radio, it must contain all of those frequencies that compose 'AM radio' - Not only the ones that might, say, pay you for exclusivity; no, you must supply the entire AM band.
That was later expanded to FM band, which again, is a reserved band of frequencies, licensed for use by broadcasting stations - if you supply FM band, you must supply it all, in fairness to those who have paid licensing within the scheme.
Since AM is the root of the scheme, how is it fair to omit it? If anything, FM, as the extension, could be omitted, or the whole radio omitted... Or supply both. It would be similar to supplying the original 23 channels in CB... which DID go on for a while - you could elect to buy either 23 or 40 channels for a while. but the root channels - the 23 - were always supplied.
In this digital age, the cost of supplying AM is a pittance if a radio is supplied. So it is not a matter of monetary value. Likewise, supplying both AM and FM in a head unit is but a pittance in the cost of a head unit itself.
So if it is not a matter of saving money (assuming FM will be supplied, AM is no real extra cost) then the motive must be something else.