PayPal's agreement requires 30 days notice of any changes, and they post the changes on their website: https://www.paypal.com/us/webapps/mpp/ua/upcoming-policies-f...
Paypal's anti-money laundering rules comply with the rules set by each state that money transmitters are required to follow. The funds frozen for AML don't become paypal's -- they're seized by the government.
It is complicated. For example, New York's Department of Financial Services "interposed no objection to PayPal offering certain of its payment processing services to New York customers without being licensed as a money transmitter" in 2002 [1]. These rules are complex alone, complex in their interrelations and subject to competing regulatory interests.
I work in a regulated financial forum. I've seen, or been relayed through regulators, accounts concerning money transmitters needing to hold funds for more than 180 days to finish an investigation per their internal policy. At that point, the contract doesn't matter--statute trumps. This might be unique to New York, but I doubt we're the only state or territory with these rules (usually, our laws are the most cutting).
If it's customer versus compliance, the customer almost always has to engage regulators or sue to make progress. When using money transmitters like PayPal to not only move funds but also hold them you're giving up a lot of the regulatory protection and venue-provisioning you get with more sophisticated services. If a business relies on PayPal, its management needs to talk to a lawyer and do more research.
NY may have been ok with paypal operating that way in 2002, but that has clearly changed:
https://www.paypalobjects.com/webstatic/en_US/licenses/NY_li...
Search for the word "notwithstanding". It usually comes in the form "notwithstanding anything to the contrary in the rest of this agreement..." Remember that PayPal incorporates--and reserves the unilateral right to change--several policy documents into its agreements.
These agreements are necessarily complicated. They're also drafted by counsel working for PayPal. At least in the case of AML, I'm reasonably certain PayPal has to, under federal law, hold funds when it suspects money laundering. Unlike banks, which have their AML policies and definitions reviewed, PayPal is relatively free in constructing this policy.