People do it, however....
Franklin electric says:
Franklin submersible motors are designed primarily for operation in the vertical, shaft-up position. During acceleration, the pump thrust increases as its output head increases. In cases where the pump head stays below its normal operating range during startup and full speed condition, the pump may create upward thrust. This creates upward thrust on the motor upthrust bearing. This is an acceptable operation for short periods at each start, but running continuously with upthrust will cause excessive wear on the upthrust bearing.
As the mounting position becomes further from vertical and closer to horizontal, the probability of shortened thrust bearing life increases. For normal motor life expectancy with motor positions other than shaft-up, follow these recommendations:
1. Minimize the frequency of starts, preferably to fewer than 10 per 24-hour period. 6” and 8” motors should have a minimum of 20 minutes between starts or starting attempts.
2. Do not use in systems which can run even for short periods at full speed without thrust toward the motor.