Well it makes sense, doesn't it? The costs for a mobile network provider are largely fixed and not really tied to consumption (it's maintenance and predictable/steady network upgrades).
Calls and SMS don't really cost the network anything (aside from inter-network fees). They're priced in such a way so that with the assumed amount of customers, they'll recoup their fixed costs. If people stop making calls and sending SMS, they lose that predicted income and have to recoup it from other sources (either the fixed monthly fees, data fees, or tacking on bullshit service charges).
Of course it makes sense; when you stick a price on a new service and find out a couple of years into selling that service that the price you put on it doesn't level with the costs you are making to maintain it. Then you need to level it; moral issues aside (some say the ISPs lured clients in with low prices only to raise them after they were in), that's the way it works. But then again, just raise the price, and do not block certain ports and sell it as a 'mobile internet' subscription or charge extra for 'certain kinds' (e.g. skype, whatsapp) of data usage.
The problem mostly was that people who were once overusing their SMS/text bundles/credits were now no longer paying huge bills because of Whatsapp. BBM offset this for providers by having a seperate BBM internet plan required.
Calls and SMS don't really cost the network anything (aside from inter-network fees). They're priced in such a way so that with the assumed amount of customers, they'll recoup their fixed costs. If people stop making calls and sending SMS, they lose that predicted income and have to recoup it from other sources (either the fixed monthly fees, data fees, or tacking on bullshit service charges).