Companies exist (only) to make money. The only reason Apple has to not sell users' data is because they think the reputational and other costs are more than the profit from selling it.
If at some point in the future, the expected costs and profits change to make it profitable, they will do it without a second thought.
If the code is closed source, the users are less likely to be able to tell whether their data is being sold. This reduces the expected costs of selling the data (in PR expenses, customers who take their business elsewhere on principle and so on).
If the code is open source, it's more likely to be more costly to sell users' data, so it's more likely that it won't be worth it even in the future.
Companies exist (only) to make money. The only reason Apple has to not sell users' data is because they think the reputational and other costs are more than the profit from selling it.
If at some point in the future, the expected costs and profits change to make it profitable, they will do it without a second thought.
If the code is closed source, the users are less likely to be able to tell whether their data is being sold. This reduces the expected costs of selling the data (in PR expenses, customers who take their business elsewhere on principle and so on).
If the code is open source, it's more likely to be more costly to sell users' data, so it's more likely that it won't be worth it even in the future.