That is an interesting observation. Every Jamba Juice I can think of is near a Starbucks.
I did find this article about the two companies in Hawaii. Here is an excerpt:
"The brands haven't been here long. Starbucks opened in Kahala Mall in
1996 and the first Jamba Juice opened in Kailua in 1999. However,
today, they're virtually everywhere, often right next to each other.
That's not a surprise, as the same local holding company, Cafe Partners
Hawaii, owns a 95 percent stake in each Hawaii licensee. Coffee
Partners Hawaii (the Starbucks licensee) is a joint venture with
Washington-based Starbucks Coffee (NASDAQ:SBUX) and JJC Hawaii LLC (the
Jamba Juice licensee) is a joint venture with the privately held Jamba
Juice Co."