For example, let's say T-Mobile's Music Freedom program (which uses packet inspection to whitelist Spotify, Pandora, and a few others so they don't count in data metering) expands to also encompass popular video streaming services: Netflix, YouTube, Vimeo, Hulu, etc.
Now a consumer could pay T-Mobile for an inexpensive plan, say $20 for 1 GB of data on a mobile device. They stream high-definition video and music every night, maybe using 40 GB of data this month. But they only pay for 1 GB, which is still available to them for email and general internet browsing.
No extra penalties, no extra fees.
That's a great deal for the consumer, since the majority of their bandwidth use is probably streaming media from these services. The free streaming is a value-add which encourages users to pay for streaming services, but it doesn't count against the advertised data they pay for.
This clearly discriminates different kinds of traffic and provides preferential treatment to certain services. But it doesn't cause general internet services to be slower. Storing the media in hard drives near cell towers may cause the preferred media to be faster.