There is evidence of this on the Sprint forums (you may have to have an account to view this):
https://community.sprint.com/baw/thread/138586?start=0&tstart=0
The relevant quote from the Sprint employee is as follows:
"Thank you for the post. This is a great question. Unfortunately, the Nexus 5 will not be available for the Sprint As You Go plans. It will need to be on either the Unlimited, My Way or My All-In plan which does require a 2 yr contract. To clarify, if you are a new customer, you will have the contract based on the plan but your upgrade eligibility will not be effected. If you are a current customer, neither your contract nor your upgrade eligibility will be effected if you purchase the Nexus 5 from the Google Play Store and activate it on your account. In both instances, there will be an activation fee of $36.
I hope this clears things up for you. Please let me know if you have any other questions.
- *Kitty
Sprint Social Care Team"