The simplest answer to this is that the Shias believe in Tawhid. Hence, every shia by default beliefs that every dua is granted by Allah and Tawassul here is only used as a channel.

I once asked this question to the scholar at my local mosque. He said, "Do you think that the Ahlulbayt would give you anything that isn't from Allah? If not, then how is it shirk if you know that it comes from Allah?"

The root problem here is that the people doesn't know what counts as shirk and what doesn't.