"Pizza is a Vegetable" is a satirical expression inspired by a spending bill passed by the U.S. Congress in November 2011, which bars the Department of Agriculture from changing the nutritional guidelines for public school lunches, such as raising the minimum amount of tomato paste on pizza to be counted as a serving of vegetable.
On November 14th, 2011, the Associated Press reported that U.S. House Republicans put forth a spending bill that would bar the USDA from changing its nutritional guidelines for school lunches, which would've required more green vegetables and set a higher qualification for tomato paste to be counted as vegetables from 2 tablespoons to a a half-cup. The article also revealed that part of the spending bill would protect the status of tomato paste on pizza as a vegetable at the request of food companies supplying the nation's school cafeterias.
According to Wikipedia, tomatoes are botanically considered a fruit since they contain seeds but the United States Supreme Court ruled in the 1893 Nix v. Hedden decision that tomatoes should be considered a vegetable for trade purposes. During this period, taxes were required for all vegetables imports and the Supreme Court ruled against the Nix family's attempt to recover back duties paid to the Collector of the Port of New York.
On November 16th, a thread titled "Congress declares pizza a vegetable" reached the front page of Reddit and accumulated over 1,200 up votes within a week. The earliest known image macro was posted by Redditor Wings144 the following day, titled "Scumbag Congress…" (shown below).
On November 18th, a screenshot of a news article titled "US Congress rules that pizza is a vegetable" with the "I don't want to live on this planet anymore" image macro was posted to the Cheezburger site Memebase (shown below). The same day, a compilation of image macros referencing the news story were posted to the Cheezburger site FAILblog.
On November 20th, a thread featuring pizza-vegetable images was posted on the Body Building Forums. On the following day, the Internet news site UpRoxx published a post titled "Meme Watch: Pizza is Apparently a Vegetable," featuring a slideshow of notable example images. On November 23rd, YouTuber Jonathan Mann uploaded a "Pizza is a Vegatable" music video (show below). Additional examples of the meme have been posted on Tumblr, FunnyJunk and Reddit. As of October 4th, 2012, a Facebook page for "When I was your age, pizza wasn't a vegetable" has accumulated 180 likes.
Many images were created parodying the proposed congressional bill, including photoshopped pizzas on a variety of plants and image macros with captions mocking the classification of pizza as a vegetable.
Search queries for "pizza is a vegetable" climbed dramatically in November of 2011 immediately after the bill was released.
 Education Week – Is Pizza a Vegetable? In School Lunches, Congress Says Yes
 The Huffington Post – How Pizza Became A Vegetable Through The Magic Of Influence-Peddling