Actually, it would waste something: electricity. Thermal printers use power to heat the paper up, which makes it turn black. A receipt with more black printing

Actually, it would waste something: electricity. Thermal printers use power to heat the paper up, which makes it turn black. A receipt with more black printing on it takes more power to print. Not all receipt printers are thermal, however: a lot are thermal-transfer, that uses heat to transfer ink from a film to the paper. That stuff looks much better, and lasts much longer than the crappy thermal-printed receipts.