Aspeaker rehearsing their speech beforehand is adhering to which western moral expectation of speakers

It is adhering to "be prepared".

Knowing exactly what you are going to speak to an audience is considered one of the primary traits of an ethical speaker, rehearsing your speech makes it sure that you are prepared for the task.  

Public speaking ought to mirror the character of the speaker and ought to be founded on an establishment of trust, uprightness, regard and poise. The speaker ought to keep away from practices like stereotyping, harming messages and copyright infringement etc.