They charge differently for using their network with a computer, because they know you're going to use more data than you would on a handset. If the FCC ever gets involved, they will probably put an end to that, but that's just how it is for now.
How do you figure $90/mo, though? $40/mo for voice, $30/mo for data, $10/mo for texting only comes out to $80/mo, and that's assuming you get those plans, and is before any discounts.