"why R22 tail rotor blades don't have a main spar. Is this common in tail rotor construction?"
DR - whilst not speaking for all TRB's, the ones we work have spars, but this can be misleading as they usually form the leading edge and may be a hollow forging (as indeed they do on a lot of Main Rotor Blades), the rest is just thin metal or composite to form the shape. The one exeception is the S76 TRB that has a removable spar onto which the paddles are bolted. I cannot speak for the Robbo blades (main or tail) as there are no approved repair schemes for them (no value either!) and we don't see them.
PM me if you want more info.
R2