As this is new functionality introduced in 8.1, till now we cannot use Rejection justification with email based approvals.
If we set Rejection Justification to Yes then email based approvals will not work.
May be you can create an idea to implement this in future releases.
I don't know if 8.1 SP1 will have it or not.
Thanks Sidh for the input.
Your observation is correct. In case if admin user set ‘Require Justification on Rejection' option to YES, then with email assignment notification user would NOT see ‘ Approve’ and ‘Reject’ Link in email message body.
I also recommend you to check below webinar recording and QA information, to understand information related to Email Based Approvals functionality.
Hope this would be useful!
As mentioned by Shiv and Sidh, this is not available in 8.x and can be considered as a limitation. Email-approvals being newly introduced, I would think that it will surely go through good and useful enhancements in future. So I would suggest to create an Idea if not already exists.