Probably because most capitalists don't believe this?
>nor did the property rights regarding their shares.
Debatable. Most capitalists would say that property rights naturally exist and that "shares" are simply an agreed upon way of dividing property rights over a complexity of assets and agreements (i.e. a business).
>Taxing large profits is no more "unnatural" than protecting the right to these profits in the first place.
True. Obviously a laissez-faire capitalists would say they're both unnatural- as they would say most, if not all, concepts created via legislative action are also "unnatural." Of course, people don't typically expend effort fighting "unnatural" injustice that benefits them- especially when money is on the line, and even more so state protected stock holder money.
>A business acting unethically but legally is most rectifiably a problem with the law,
Since when is it the legislature's job to enforce morality?