It is a small handful of natural persons (or corporate board or executive team) deciding for the entire corporation full of people who have zero say that the fruits of their economic output (under the auspices of the corporation) are being used to support a particular political party or candidate.
If we look at individual political campaign contributions, then it's one person deciding that some of their hard-earned cash should go to support a particular candidate or cause. But with a corporate structure, it's five people deciding that the collectively-earned cash of thousands of people should go to support a particular candidate or cause.
To put it another way, an individual campaign contribution is "one person, one vote". A corporate contribution is "one person, many votes".
And no, most people do not have the realistic option to quit their job because they don't agree with the political contributions the executives have decided the company will make.
> Should government also have a right to restrict speech of individuals if they are rich and powerful enough?
Governments already do this: individuals are subject to political campaign contribution limits[0]. This unfortunately gets muddied by PACs and the like, which was the issue at hand in Citizens United.
[0] https://www.fec.gov/help-candidates-and-committees/candidate...