They could have an option to sell a power supply as an addon item. However, unless it's a well tested and known brand (if not built in house), you can get fires and potential law suits as we've already seen from bitmaintech.
It is a really tough one for ASIC manufacturers.
1. An autoranging PSU 100V-240V rated @ max 550W will only provide the max power at the top end voltage, and though the PSU may be able to power your ASIC rig from the lower end voltage, it will be stretched and most likely die on you gracefully or probably go out with a bang! So they can choose to put at autoranging one rated @ max 750W, but this is an over-specified PSU for the higher voltage countries.
2. Most users want to (and do) OC their rigs, so a lowly specced PSU will most likely be overstretched to the expense of the manufacturer.
3. Better to specify the power needs of a rig and let the customer source a PSU from the local market to power their rig.
Having said that, redistributors tend to stock and offer PSU's matched to the rigs they sell that do not ship with manufacturers' PSU's. At the end of the day, whether you buy a rig with a manufacturer PSU or from the ocal market, you will still pay for the rig. in the case of the latter day bitmain shipping charges, I'd err on the side of sourcing my PSU locally rather than have the manufacturer whack me with a great big shipping bill!