That text is there because Apple doesn't know what your cellular contract says or what fees may apply. Always check with your provider for any extra fees. While they don't have to shove it in your face, they do have make the information available.
As SciFan says, Verizon no longer charges extra to activate the hotspot feature on your iPhone or iPad (which means it's rolled into the plan, not really free). However, you could still be charged extra if you exceed whatever limits your Verizon has. To avoid this go to your MyVerizon page on the web and make sure you have your data limit alerts set to warn you. You can choose SMS messages to your phone, email, or both.
Verizon also has an iPad and iPhone app that you can used to check your current data usage (it always lags a bit, so be careful when approaching the limit)
In the iPhone version only, you can add a widget to your Today notification view. It lets you see your current data usage at a glance.