IP blocks allocations are assets but they are granted by the RIR. [1] Your allocation is given to you in exchange for being a member of the RIR. You don't actually own the IPs, but you do have exclusive rights to them but the RIR can ultimately decide to revoke your rights to them for violating RIR policies.
RIRs do allow you to transfer your IP blocks to other organizations but you can only do so if the receiving organization proves to the RIR they have a good reason to hold those blocks of IPs. This valuation is based on what a typical IPv4 owner receives in exchange for that transfer of IPv4 addresses.
Just like any assets which you hold a lot of, if AWS dumped their IP addresses on the transfer market, the price of IPv4s would likely fall significantly so I doubt they could actually get that price for all their IPv4s.
The precedent for this came from Nortel's bankruptcy where they sold IPs to Microsoft. Some tried to argue that Nortel should have given up the addresses for free but the court ruled that IPs are property.
