An overview of USB says low-speed data is “clocked at 1.50Mb/s with a data signalling tolerance of ±1.5% or 15,000ppm” so you can clock it with a [ceramic] resonator instead of needing a crystal.
AVRs’ internal RC resonators are mostly not quite this precise; even at a fixed temperature and voltage and after user calibration, they’re rated to vary by 2%, at least on the ATtiny2313. But on the ATMega series (at least the 48/88/168/328 series used in the Arduino), they’re rated to ±1% under these circumstances. So a resonator is needed to bitbang low-speed USB on the ATtinies, but maybe not the ATMegas.