It's not dirty: it keeps incentives aligned because the shareholders have common goals, and the terms are stated clearly upfront.

But what we have here is not only that but also a prohibition on sale to an outsider. So not only "we can intercept the sale at the same price you offered to someone else" but also "we can block that sale even if we don't want to participate in it".

“The attached document is part of the Startup Forms Library made available by Clerky as part of an initiative with Orrick and Y Combinator to streamline startup legal documents. By using or viewing the attached document, you agree to the Terms of Use for the Startup Forms Library, which can be viewed at https://www.clerky.com/site/form-terms.”

Transferability:

“You may not transfer this Option except as set forth in Section 6 of the Stock Option Agreement (subject to compliance with Applicable Laws). You must obtain Company approval prior to any transfer of the Shares received upon exercise of this Option.”