Why do the host cells containing iC3b fragment not undergo phagocytosis?
There's a relatively new concept of self-associated molecular patterns (SAMPs), which much like DAMPs and PAMPs, help immune cells make a decision in a slurry of signals. There's at least a fair article on SAMPs here, and to go even further, the suggestion that tumor-associated SAMPs can dampen immune clearance of cancer cells. 

In my head a few things are going on: 

(1) The opsonins are potentially binding to everything. Of course, there are surfaces like ECM that don't have any receptors or patterns that you don't want getting damaged, and there's at least some evidence that complement clearance by factors I and H (paywall) may contribute to ablating that effect.

(2) The actual targets for the immune system are expressing patterns that specialized pattern recognition receptors on your phagocytes and so forth can recognize.

(3) Self cells are expressing receptors and patterns that tell the very same cells there's no threat, either through direct inhibition or degradation of kill signals.

Remember that when self cells are damaged, the expression of DAMPs help macrophages and the like come clear them away, so there's obviously a way for self cells to control that effect.