Most is possibly an overstatement here. Many cheap printers save a few cents by skipping the FET for the fan and hardwiring it, or don't assemble them correctly, but plenty of E3D-style hot ends with cooling fans are installed by printer manufacturers who wire things correctly or are aftermarket mods or on custom builds and are properly wired by their owners.
Please do not take the following as an attack, as it's not, but rather as the advice of someone who knows a bit about failure modes and is concerned for your well-being:
This may have worked so far in your case, but it's not a reliable solution to assume that powering a printer control board from an RPi, even when not heating, etc., won't cause a power event on the Pi or cause the controller to wind up in a dangerous state, especially since supervisors and watchdogs are often not properly implemented to ensure resets occur if a brownout condition causes a hang. Please keep in mind that brownout conditions can lead to pins in unreliable states, which may do something stupid that you'd never expect or intentionally experience, like turn on your power supply and heaters with no PWM or monitoring, in the case of your particular setup.
This setup is a fire hazard because it could cause heaters to turn on without thermal runaway protection, due to a hung MCU. The likelihood of this happening is high enough that similar failure modes have led to specific safety regulations to avoid them in industrial automation, automotive systems, aeronautics, etc. 3D printers and RPis are not designed to these specs and are far more susceptible to these failure modes than your average car (look up people being injured or killed due to problems with their automotive electronics/indicators - it happens, even with these standards, and leads to recalls to reprogram or replace ECUs, etc.; also look up posts on here where RPis and/or USB peripherals misbehave and it turns out to be a power issue - it's quite common).
Your printer power scheme, while creative, is unusual and contrary to good engineering safety practices. Get a Sonoff switch or similar for <$10 and prevent a sad story. If you don't believe me, read @foosel's posts about why OctoPrint checks if a printer's firmware has thermal runaway protection turned on and search the forums and follow the links to reports of houses that burned down.
Hack a Sonoff and save a life today!