One of the big drawbacks about the iPad 2 is its camera; identical to the iPod touch 4th generation's camera, it only takes photos at a 960x720 resolution, paling in comparison to the resolution available from the iPhone 4's camera, if not all iPhone cameras period. This is such low resolution that it is actually smaller than the iPad 2's screen resolution of 1024x768. This means that users should not expect to be taking fantastic photographs with the iPad 2; and apparently very few people are.
According to Flickr's stats, there are an average of 36 users per day uploading photos to Flickr from their iPad. Compare this to the over 4000 that upload daily from the iPhone 4 (which is trending toward becoming the most-used camera on Flickr, period), over 3000 that still upload photos from the iPhone 3G, and just under 2000 that upload from the iPhone 3GS per day. In comparison, the iPod touch 4G, which has an identical camera as the iPad 2, gets 455 users that upload to photos to the site per day, which is enough to make it the 5th most popular 'cameraphone' on Flickr; stats are not available for the original iPhone on Flickr.
Granted, there is a definite possibility that the numbers are being skewed by Flickr's userbase; it may skew more toward photography enthusiasts than the casual photo taker, and the iPad 2's camera is one that few if any serious photographers would actually use. The percentages may likely be higher on Facebook and Twitter photo services, where casual and convenient photos are more prominent than well-prepared shots, where a higher-quality camera would likely be used.
However, what is clear is that iOS users have definite options to upload photos from the phone to Flickr, as shown by iOS cameras being 4 of the top 5 cameraphones on Flickr. The iPod touch's inferior camera is still popular enough to make it notable among cameraphones on Flickr. The conclusion is that tablets just may not be devices that users consider as regular cameras. iPhones and iPod touches are devices designed to be carried around in users' pockets, and can be used to take photos when an opportunity arises. The iPad is usually carried around in a case or a bag, and may not be as immediately available for casual photo taking, and its inferior quality camera dissuades taking high-quality shots as well. Apple may have known this and only put a rear-facing camera in for posterity, rather than for actual usability, because no one is using the iPad 2 to take photos.