The idea of Rava is to learn worship not according to the way of that idol from "he will bow" instead of "he will sacrifice" and that would tell us all kinds of service of honor would be forbidden--even not service according to the way of that idol.
Then Rav Acha asks on Rava from the statement of Rabbi Elazer who says: How do we know one can't sacrifice to Mercury? From the verse, "So that they shall no longer bring their sacrifices to the goats."
The question of Rav Acha is this: If we already know from "bowing" all kinds of service of honor, then why do we also need this other way of Rabbi Elezar to tell us less than what we already know?
The Gemara answers: The statement of Rabbi Elezar refers to when one sacrifices in order to make G-d mad, not to serve idols. Now I think we can all agree that this answer sounds strained. If we look at the verse we can see that the idea is God says "I am making this law that they bring their sacrifices to the tabernacle in order that they should no longer bring their sacrifices to the goats (idols)." Surely they were not bring their sacrifices to the goats (idols) in order to make God mad, but rather to worship idolatry. And since God is making this parallelism, it would have to mean: They should no longer bring their sacrifices to goats (idols) in order to make me mad and rather bring their sacrifices to me to make me mad. [Of course, you could say it really is anti parallel--and God means rather: They should bring their sacrifices to me to make the goats (idols) mad.] It does not matter anyway because all I am doing is showing how the answer of the Gemara is anything but obvious in order to build up my question that they Gemara could have proposed an alternative answer.
So my question is this. Why did not the Gemara say simply that Rabbi Elazar refers to Mercury and other idol that are worshiped in ways of dishonor, and Rava was referring to idols that are worshiped in a way of honor?
That is the same way the Gemara divides worship not according to it way into four parts -worship of dishonor and idols of dishonor and worship of honor and idols of honor so do the same here.
One answer here is you need only Markulis [Mercury] and you know idolatry of honor by a a fortiori. But that can't work because we don't make prohibitions based on a fortiori.