Trivia: One of the severed heads on a spike is that of former president George W. Bush. Before shooting the scene, George RR Martin asked writers and producers David Benioff and D.B.Weis to have a cast of their 3 heads to be put on the spikes, but for budget reasons they opted to get a box from HBO's warehouse with used severed heads. They noted that one was Bush's, but they put a wig on it and got away with nobody noticing. In the Blu-Ray commentary the producers revealed the story, and got a lot of criticism from the right.
Answer: This is explained more in the book. Khal Drogo and his bloodriders do intend to help Viserys take the throne, however they do things in their own time. This is what causes Viserys's frustration. In the book, Illyrio Mopatis tells Viserys that he has already waited his entire life to claim The Iron Throne and that waiting a few more years isn't that big of a deal. The Dothraki are a nomadic tribe, which is largely why they have no interest in invading and sacking King's Landing. However, since Khal Drogo eventually grows to love Dany, her assassination attempt by the Westerosi compels him to seek revenge.
Phaneron ★