Check page 32 of the tiger Cat II manual.
The spindle bearings are not sealed but there is a separate seal below the lower bearing and the cap on the upper.
There are two reasons why there are plugs installed and not grease fittings. 1. Some operators bump the front into objects and can break off the grease fittings. 2. Grease guns are capable of putting out high pressures, so to prevent the cap from being blown off, Scag recommends removing the cap, removing the plug and installing a grease fitting and pumping grease until it is noticed at the upper bearing. Then reinstall the cap, remove the grease fitting and reinstall the plug. Some operators leave the grease fitting installed and this is OK to a point as long as the cap is removed before greasing the spindle and reinstalled after.
Some operators have drilled a small hole in the cap to allow the air to vent and grease to show and leave the grease fittings installed to reduce the time spent during a 200-500 hour lube.
I have noticed that some of the flat free front wheels do not have a grease fitting, not sure what type of bearing they have in them as there are several different suppliers of these wheels. The pneumatic tire wheels have two straight roller bearings which although frequently greased, can still get corroded and run rough.
The spindles on Scags are setup so there is some turning drag on the spindles, this improves their tracking during machine operation.