There are two sides to this seeming contradiction. Joemikeb's comment covers a reason why APC (and other brands) might caution against using UPSes with printers. That said, not all printers are alike in power consumption, and many inkjet printers can be used safely when running on UPS battery power (within the limits of the UPS). Laser printers, however, should not be used this way, because of the power surge associated with preheating the fuser assembly on printer wakeup. In the past, CRT monitors would be excluded from UPS backup for a similar reason.
On the printer side there are non-electrical considerations. Among other things, turning off a printer with its on/off switch returns the printhead to its resting position, which protects the ink nozzles from clogging by drying out. This is particularly relevant to Epson printers, which (used to) have a fixed printhead, instead of one built into the ink cartridge. Fixed printheads that get irreversibly clogged render the entire printer useless. 'Cartridge printheads' are much cheaper to replace, should that happen.
Why did I say 'seeming contradiction'? Largely because potential problems can be minimized if not avoided altogether. When UPSes are only used for qualifying equipment whose power consumption is within their capacity, there is only an issue when a printer is ON, and NOT hooked up to a UPS when the power goes out. In those cases it suffices to power cycle (on/off) affected printers to park their printheads properly when power returns.
