im pretty sure its due to vibration, you wouldnt want the vibration wearing the bushes so the grease makes them slip instead of rub
 
The grease they supply is just standard copper grease, so if they start squeaking in the future you can just get normal copper grease to re-lube them.
 
It isnt standard copper grease its finer

Standard copper grease on bushes can wear the bush

Superpro bushes actually use clear grease (cant exactly remember the type)