51% of a company is not some magic ticket that allows you to do basically anything (including screwing the other 49%). It just doesn't work that way. There are certain thresholds that allow you to do more and more things but if you're in control of a company you still have a fiduciary duty to the other shareholders, even small minority shareholders. This is why minority shareholder lawsuits are a thing. This is what makes the entire corporation system work otherwise the 51% would be constantly screwing over the 49%.
Take your example: if you buy 51% of the shares for $50/share and then try and merge at an effective $25/share. Well, that's illegal because you've failed your fiduciary duty to the 49%. You're effectively trying to steal from them.
Also, you can't just acquire 51% of a company. There are rules about that too. There are thresholds here too. IIRC more than 5% and you have to inform the market of taking a large position. More than (IIRC) 10-20% and you have to launch a formal takeover offer. There are rules about how that works too.