@mavica_again Not a citizen of USAtania myself, but I looked into this quite a long time ago, and it appears to be a weird mixture between having anti-trust laws, but not enough consumer protection laws. So it's not blocked by false advertising laws, but also a workaround to minimum price arrangements etc.
Also quite huge in the electronics sector of the 80s/90s. Margins got a lot lower (e.g. by Japan entering the market big time), and everything became more data-driven. So if you mailed in, they got your data, and if you didn't, they "got" your money.
And in general, doing things via mail was quite popular in the US, probably due to its size and sometimes low population density. Mail-in rebates meshes nicely with mail-order catalogs or those registration cards you found everywhere.