Think of it like your house. You have a single connection to an ISP that provides a fixed amount of bandwidth, and many people have several devices in their homes connected to that single service, all sharing that bandwidth. The same goes for cell towers. Verizon needs to figure out how much data will be accessed at peak times by how many users and devices and make sure that there is adequate bandwidth at the tower to provide the service without slowing everybody to a crawl. That backhaul (i.e., the connection from the tower out to Verizon's network) isn't free, and the more bandwidth that needs to be provided, the more it is going to cost.
Of course Verizon threw this argument out the window when they decided to allow hotspot for free on the new iPad... however, you get the deal that you get.
Your laptop is using bandwidth at the same time as your phone (remember that the phone is a multitasking device.) You are using bandwidth that Verizon has to budget for.
I know when I bought my Bionic it was very clear from anywhere I researched (and the saleswoman confirmed) that hotspot features cost extra (and comes with its own 2 GB data cap - it doesn't count against your existing data package). The hotspot can also be turned on and off on a per-month basis, so you can use it only when you need it, without any penalty, change in contract, etc. - all this was explained to me when I asked to confirm my understanding of what was on the web site (I was glad that I asked - I didn't know that you could turn hotspot on and off like that.) If you failed to research this or ask about it when you purchased, that's not Verizon's fault.