No, that's correct.

A tablet on unlimited data is $20 IF it's an additional line to accompany phone lines.

Example:

1 smart phone on Go Unlimited is $80, adding a tablet would be an additional $20.

When you have no phones and only data only devices, the first one gets charged the cost of what a phone line would be.

Example: 

Tablet 1 on unlimited is $80

Tablet 2 on unlimited is $20

This is why you don't sign contracts for service.