It's a security feature to protect you, the owner of a locked device, against personal data theft in the event you lose the device or it is stolen.
In order to root the N7 you need to unlock the bootloader. Nexus devices are "developer" devices that come with the ability to very easily unlock the bootloader, it only takes a simple ADB command to do so.
Once the bootloader is unlocked a user can achieve elevated system access even if that user is a thief or someone else with nefarious intent. They can easily bypass your pin or swipe lock, find ways to gain access to encrypted data and what not.
The mechanic that protects you is a hardware mandated complete device wipe when the bootloader is unlocked. The wipe cannot be bypassed, it is hardware coded and when it happens it is thorough enough that recovering previous data is extremely difficult if not close to impossible.
There are ways you can save app data without root at first, I highly suggest Helium.
https://play.google.com/store/apps/details?id=com.koushikdutta.backup
Helium will require a computer to enable app data backup on your non-rooted device, Helium Desktop is the program and available for Win, Mac, and Linux.
ClockworkMod Carbon
Get and install Helium on your N7.
Get the proper version of Helium Desktop on your computer.
Launch Helium on your device, it will guide you through connecting to your computer and enabling USB debug mode and PTP.
Once your device is set it will instruct you to launch the desktop client. Run the client and it take a minute while it detects and enables backups on your device.
You should get a completion message.
Now put your device in PC Download mode. In helium click settings and PC Download.
Navigate to the IP address shown on your device screen using Chrome or FireFox.
Now checkoff all the data points and apps you want to save and click Start backup.
Be patient.
OR
Use Helium on the device, create the backups you want, connect the device to a computer and then transfer the backup files off the device.
Your pictures, music, videos, downloads and other user managed data will not be saved by Helium or any other backup. You must transfer those files off the device manually by connecting to a computer in MTP mode and transfer them off.
Helium, like any other backup utility, is a user dependent app. Make sure you understand it to ensure your backups are viable. Make a test backup of a single app, delete the app from the device, and restore it with Helium to make sure you are doing it right.
Once you are sure your backups are viable and off the device, along with other data like pics music movies and whatnot then you can proceed with unlocking your bootloader.
Once you are unlocked and rooted reinstall helium on the device. As you are now rooted Helium does not need to connect to the computer, just grant it root access.
Go back into PC Download mode, fire up your web browser, and drag n drop the backup file in the restore box and it will do the rest.
OR
Restore the carbon/helium backfolder back onto the device where you found it and launch Helium. Select your data to restore and way you go.
Once all that is done, make a nandroid of your device in recovery and keep it safe. In the event you need it, restoring the nandroid will take the device right back to the minute you made it.
Im sure you will have questions, Ill be here to answer them so fire them off.
Make sure your device is connected to the computer directly, do not use a USB hub. Also make sure you are using a high quality cable, preferably the one that came with the N7.
