Is it reasonable for an exchange to take a passive stance towards updates from a platform like Waves?
An exchange has to protect its owners and client against foreseeable losses. A network fork can create risks of loss of funds for an exchange.
Anything that results in the loss of funds can potentially shutter an exchange. Therefore it is up to an exchange to minimize risk.
Cryptopia is risk adverse. They didn't list Ethereum until the fork was well established. (Technically Ethereum is a larger fork from Ethereum Classic)
When an exchange receives funds it is deposited to the exchange wallet but becomes funds held on the exchange by the user. It is often credited after a short number of confirmations. The user can then use the funds for trading. e.g. User1 deposits 10 Waves into wallet.
10 waves is credited to exchange wallet. User1 has 10 Waves in funds on exchange database. User1 trades 10 Waves for 1 Ethereum with user2 and User1 withdraws the Ethereum. User2 has now 10 Waves in exchange Database. The fork results in loss of the 10 Waves = exchange wallet is now 10 Waves short.
When users buy and sell it occurs on the exchange database - not on the coin blockchain. So any losses due to a fork are sustained by the exchange. They may be able to track it back to a user - but is it recoverable ?
Is it necessary to delist the assets as well?
The tokens or assets rely on the proper functioning of the network. The risks of loss due to a fork are the same.
I have spoken with the developers of the site but I have never met them in person. Other than expressing my sadness at losing Waves trading on Cryptopia I haven't discussed the specifics of the removal. This is my personal opinion only.