One new incentive is the falling transaction costs and barriers to entry for entrepreneurs of every age.

FORBES: Boomers are Driving a New Entrepreneurship Boom

Most got their start on Taobao, where barriers to entry are almost nonexistent because the site collects no membership or transaction fees.

FORBES: 11 Tips For Success For Startups Looking To Sell On China's Taobao

And, the department said that any potential entry from a new mobile wireless telecommunications services provider would be unable to offset the transaction's anticompetitive effects because it would be difficult, time-consuming and expensive, requiring spectrum licenses and the construction of a network.

ENGADGET: US government files to block proposed AT&T / T-Mobile merger (update: companies respond)