I tested Viber a couple of years back, and their response to my question if they encrypted any of the traffic was "no" - which is when I dropped it from apps I use.
These companies are not interested in protecting you (you're not a customer, you're a number - remember, it's "free"?). They will do as little as possible to get some functionality working and off it goes, and with the volume the market offers that's enough for good profit - most people are still not aware what a risk they take when all their traffic can be intercepted. For Joe Public, it will take a dramatic event for this to be seen as a problem - after the Snowden files dry up I give it 6 months and we're back where we started. However, to be fair to Viber, to my knowledge they have never claimed to encrypt traffic - I asked them this a few years back when I could not find it in the online docs, and I got a reply they didn't.
Now, let's assume Viber gets asked for crypto. The way most of these outfits work, they will slap on a library and call it "safe", because to do it right, encryption and security need to be part from the design from the ground up which is effort they will not expend - also because it gets them into trouble in the US when there is a official demand for data.
So, am I surprised about this "discovery"? No.