I think the edit history is important. Not only are there many organizations, such as law firms and accounting practices, that are legally required to be compliant in various and often exhaustive ways, I also think that Yammer should do whatever it can to encourage tranparency.
Usually, edits are pretty benign in nature, but sometimes they can reflect policy change, language change, etc. We just can't know, so I appreciate the option to see what those changes are when I can.
@Tom Noels Facebook has a similar feature and also does not let you hide the edit history from others.
I'm going to change the best response to the one that actually answers your question about why this can't be disabled. In this case, vast majority of customers do want to see the edit history. Providing a toggle would take additional effort/resources on the part of Microsoft and that effort may not be worth it when a small minority want the feature.
That said, if you're a Microsoft customer and this feature is important to your organization's use of Yammer you should escalate the concerns through your account team (and via UserVoice of course).