A smart phone, by way of contrast, might use about 500 Mbytes a month. So demand-sensitive mobile networks might well be leery of encouraging use of the network by devices that create an order of magnitude greater demand than a typical smart phone.
At some level, even though mobile service providers might like to have the additional revenue and accounts mobile PC connections represent, it is inherently more difficult to maintain a user experience free of cap overage fees, which irritate users.
You might argue that service providers can simply offer bigger buckets of usage, but some might argue that will confuse many users.
Mobile PCs have the highest average monthly traffic volume per subscription over 3G networks (global average at 1 Gbyte to 2 GBytes), followed by tablets at 250 Mbytes to 800 MBytes. Smart phones typically use only 80 Mbytes to 600 MBytes.
The point is that a 4-Gbyte or 5-Gbyte data plan so outstrips typical usage of smart phone and tablet users that there is little chance most people would ever be faced with an unpleasant overage charge. That clearly is not the case for PCs connected to mobile networks, which rather easily can consume all of a 4-Gbyte or 5-Gbyte data plan.