For an Opt-In type of service, wouldn't any TOS pretty much be the final word ?
...as in, "By choosing to use this service, you agree to all terms and conditions...otherwise, don't use it" ?
Not necessarily. While a contract is essentially a private agreement between two parties, it operates within a broader legal framework and there may be legislative requirements which over-ride the contract terms. Many types of businesses are required by law to not co-mingle funds. That requirement doesn't need to be written into a contract to apply. If they have a legal duty to comply with AML/CTF requirements, that duty exists whether specifically stated in their ToS or not. Disclaimers don't necessarily protect businesses if they're operating outside of the law. You can put "this is not financial/legal" advice on your website, but a court may find that the manner in which you were acting means that you were
providing financial/legal advice. Product liability is another area where an express or implied contract may not hold up. So is personal injury (in my jurisdiction, workplace injury is strict liability - it doesn't matter if the employees caused their injury by extreme recklessness, the employer is still liable).
The "fine print" in a contract can matter a lot but just because something is written into a contract doesn't mean it's lawful - and just because a right or responsibility isn't written into a contract doesn't mean it doesn't exist.