Though the article doesn't explicitly state it, I believe the issue is this:
The capacity taken up by voice is fairly flat (i.e. it's not really growing; people aren't making noticeably more calls), whereas the capacity taken up by data is growing rapidly. The networks are constantly expanding their infrastructure in order to accommodate the growing demand for data.
So far this trend has not been truly reflected in pricing - the biggest cost to the end-user has been the call-time, rather than data. With 4G the data demand is expected to accelerate further, and so networks are moving to a data-defined pricing, which more accurately reflects the strain placed on their network.
In principle I don't have any issue with this pricing model - in fact I see it as inevitable. My issue is with the data-pricing levels that EE are offering. On a 4G connection, 8Gb/month should (in my eyes) be a mid-level contract - not one costing £56/month and only available when you buy a handset at the same time(!). I'd like to see plans going up to (say) 32Gb/month - in this way the service could be used for the purpose it was intended (high-bandwidth media consumption like HD streaming).