Talk to any startup about their early days. They took legal risks, knowingly or not, that would make a BigCo lawyer sick to their stomach. No startup launches on Day 0 fully compliant with every regulation and with best-in-class user data protection, etc.
If you're a startup within the big company your legal risks are the big company's risks. And they're 100x worse because everyone knows if they sue you they can sue the parent company and get paid, unlike when you sue a near-bankrupt startup.
If a big company wants a startup they need to break off a chunk of cash and start a separate company. Not a subsidiary, a totally separate company in which they happen to be the first investor. At which point that company will lose most of the BigCo advantages such as infrastructure and talent.