I will give you credit for acknowledging the "backward"/"modern" difference. The most important difference is that in the European "client regimes" (to use your language), they at least purport to maintain a semblance of democracy and human rights standards for their constituents. In that respect, call the convergent evolution towards the American-style government policy, a "client" or "vassal" state type of relationship if you'd like, but at least it's one in which (at a minimum) a superficial level of fairness is maintained (i.e., in contrast to sharia law countries).