Abu Abdullah (as) said: Bani Umayya initiated the teaching (taalim) of Emaan to the people but did not launch the teaching of Shirk (polytheism), so that when they do carry them upon it (izaa hamalu hum), they would not be recognizing it (lam ya’arifu hu).
[Al Kafi V 2 – The Book Of Belief and Disbelief CH 180 H 1]
There are two types of Shirk – the Shirk of Worship (Ibaadat) and the Shirk of Obedience (Itaa’at). The Shirk referred to here is the Shirk of obedience. Bani Umayaa never educated the people about this type of Shirk. The people thus got involved in it without realizing that they were performing it. In the same way, in the Usuli system, the Shias are unknowingly involved in the Shirk of obedience (of their mujtahid) and never realize it.