Yep - I take it you're coming from an older plan or a contract? The new ones have different line charges based on the type of device and it basically covers the "cost" of having a phone number and services other than data assigned to the device:
Tablets and wearables are $10
Smartphones are $20 on the newest plan, $25 or $15 on prior plans (depending on data size)
Basic/Messaging phones are $20 (I think this is mostly to penalize people into moving to a smartphone)
Hotspots and laptops are $20
Wireless home phone is $20
Wireless home phone with internet is $30
Some have the argument that if they buy data, they should be able to use as many devices as they want, but AT&T does have to provision a number for every device and manage that in their system, so there is some overhead. Furthermore, that charge is the voice & text portion of the bill for smartphones (so it makes sense to charge a bit more than an iPad or other tablet)
If you had an on-contract device, I think AT&T charged $40/month and that basically covered the "line fee" and the cost of paying off the phone subsidy together. Unlike Next and buying outright, you'd normally pay at least $199 up-front for the current model, so Next with line access will be a bit more than the old $40/month charge.
Verizon does it the same way. T-Mobile has a model more like Cricket where they charge more for the first line and then subsequent ones usually get cheaper, but the grand total every month for service should be fairly close.