Identify the unfair aspect with respect to the sentence extracted from an online Terms of Service:
```
you may be required to be a registered user in order to purchase some services . 

```
. OPTIONS: ['Contract by using' 'Unilateral change' 'Choice of law' 'Content removal'
 'None' 'Jurisdiction' 'Limitation of liability' 'Arbitration'
 'Unilateral termination'].