Going to the website that is data mining you and putting your personal details such as phone, address, full name and email feels counter intuitive. Not sure I feel comfortable doing so.
That's definitely shocking at first glance, but it is Transunion. They're not just a random data reseller, they're a pretty major institution that probably has your SSN already.
How else would your record be found and marked for removal, magic? They already have the information on you... what do you risk by putting it in again?
Assuming malicious/unethical intent, wouldn't the act of submitting that form add some non-trivial pieces of data about yourself? It lets them know you value your privacy and that you possess a certain degree of computer literacy. It also lets them know you (still) exist (the certainty factor of your profile increases).
It does seem, from a maximum-paranoia standpoint, like this confirms the current status of your information, adds any pieces they were missing, and provides an indication that your data is extra valuable.
After all, you're tech literate (suggesting income) and privacy aware (suggesting your information may be scarce)!
(I assume most of these groups are more honest than that, if only because data reselling is legal but that wouldn't be.)
That would be a serious overestimation of their capabilities. Even the most banal of their data are riddled with errors. Thesw companies talk a good talk but their technology moves at the speed of smell.