Why does changing the tracking template at an ad level send the ad for the approval process?

[Eternal Fool]

From the documentation:

Making changes to your tracking template at any level—except at the ad level—won't send your ad through the approval process: your ad will continue to serve, uninterrupted. If you change the tracking template at the ad level, the ad needs to go through the approval process.

Why did google add this limitation at an ad level? What is the thought process behind this?

Thanks

[Josh Radcliff (AdWords API Team)]

Hi,

Ads in AdWords are immutable, so in order to change the tracking template (or any other attribute besides the AdGroupAd.status) you have to REMOVE the existing ad and ADD a new ad with the new attributes. This triggers a new approval process, since the ad will be a new ad.

Thanks,

Josh, AdWords API Team