just remember that credit karma makes all it's money from selling you credit cards... so by using them to fill out your taxes you give them all the info on your income info to further "personalize" these ads
More personalized ads seem to be a win-win situation. If I'm already seeing credit card ads I'd like to at least see ones that are relevant for me.
Are you certain that Intuit and others aren’t doing the same thing? That is, using your data to gain insights?
After logging in to TurboTax (for the first time in a tax year) it asks you if it can share your data with its parent company (Intuit). Declining works and doesn't have any impact on TurboTax. However, based on how the UI for that looks like it might not be obvious for some people that data-sharing is optional.