Only because their web client works via the phone, and their whole service is based on having a phone number; they should do a more commonplace username/password based logins (with maybe two-factor authentication on a phone) and have a regular, proper client. It's probably the biggest chat tool out there; surely they can afford to build a native client or two?

As Cthulhu_ says it's more to do with the design of their app than iOS. Now that they have the clout of Facebook behind them they should be able to come up with a solution.