Which laws prevent a corporation with a 51% stake in another firm from exploiting the minority shareholders?

In a case where no corporate law exists, then yes, a majority voting bloc could flex its power over minority shareholders and do things to their own personal benefit [by electing a majority of the Board of Directors, who then appoint a CEO friendly to the majority bloc]. A simple example would be to enter into a contract with a related party supplier at exorbitant fees, and siphon off all profits of the company.

However, most jurisdictions do have protections for minority shareholders, and the US is no exception, though specific corporate law will vary by state. Here is a decent writeup of those protections as they apply to Virginia corporations [I am not a legal expert, but quick review of this page seems to show no immediately obvious errors] https://www.generalcounsellaw.com/minority-shareholder-protections/ . The key item relevant to this discussion is that the Board has a 'fiduciary duty' owed to all shareholders, meaning they cannot enrich themselves through abuse of power, and they cannot do something against the general financial interests of the minority shareholders.

Note that in your example, forcing the sale of minority shareholder interests in a corporate takeover wouldn't necessarily be a breech of such fiduciary responsibility. Generally speaking, if you were 'forced' to sell your shares for higher than their market value, then it would be difficult to argue that you have been cheated by that action taken by the board. My general understanding is that, depending on state and country law, you might need to prove that such an action was a breach of fiduciary responsibility [something along the lines of 'they undervalued the company and actually my shares were worth more than I got']. See further discussion of that here [again, no affiliation]: https://smallbusiness.chron.com/force-shareholder-sell-stock-66789.html .