NEC/CEC (as applied in respective jurisdictions) not an issue here since the devices are being used as rated and load isn't considered continuous.If anyone was for some reason thinking of actually doing this (no, you aren't), keep in mind this might violate electrical codes in the US.
For a 15 Amp circuit (not a 20A kitchen plug etc), the NEC limits a continuous load to 1440W.
15A * 120V * 80% = 1,440W
This isn't likely to be a practical issue, so whatever.
Otherwise, it seems like they accounted for a worst-case max draw on the whole stack of chargers leaving 200w of headroom per standard 15A breaker (typically serves a few general use outlets).