It has become common for mobile carriers to give "free" data for certain things which is called 0-rating and many other similar terms. It is usually because those things are popular and use low amounts of data/low steady streams of data that don't cause big delays for other users on the network, or because someone else is paying for it (like facebook and wikipedia paid carriers in India for their services to be "free" to the end users).
The problem with this is that it is giving an unfair advantage to everything that is 0-rated and can actually encourage people to use more bandwidth than what they really want to use. For instance, if streaming audio (pandora, spotify, etc.) uses your data cap but streaming video doesn't then you will be much more likely to stream music videos from youtube even though it uses ~5x the bandwidth or w/e.
If carriers want to encourage us to use low bandwidth then they should just sell data rates like wired ISP's do and use bursting. ie. if you haven't been using the network for a while then the first 10MB can burst at 100mbps but then it will be throttled to 2mbps or w/e and if you pay extra then it will be 3mbps, 4mbps, etc. Then when people first start using the connection it will load things very quickly but if they continue using it then it will be at a low rate that doesn't affect others much.