Many corporations find the public relations of buying an Earth Day celebration to be worth the investment. They could easily see similar value in purchasing a Green Party nomination. A Labor Party would not be immune if it won ballot status. A union bureaucrat could enter a Labor Party primary for the purpose of winning and then urge a vote for the Democrat in the general election. Disenfranchisement of political parties is not universal.
Most countries allow parties to determine their politics and candidates.
In the U.S., this was the case until the late 19th century. At that time, Democratic and Republican Party bosses made sure that their pet candidates were nominated at state conventions. Progressives and populists of that era proposed that a solution to top-down control would be forcing parties to select candidates in primary elections. Unfortunately, progressive success in changing American election laws became a classic example of a solution being worse than the original problem. Despite the propaganda that permeates history textbooks, forcing primaries on corporate parties did not democratize them.