No. In the US, many (most?) think tanks are 501(c)(3) nonprofits -- NCPPR included -- and cannot make lobbying a primary activity [1]
More broadly, think tanks are groups of like-minded people expressing their opinions, typically about how governments should be run. The fact that they accept donations in order to be able to work full time on the cause and to extend the reach of their advocacy, and that many of those donations come from individuals or organizations who stand to benefit if some of the think tank's desired policies are implemented, does not make them "legalized corruption".
More broadly, think tanks are groups of like-minded people expressing their opinions, typically about how governments should be run. The fact that they accept donations in order to be able to work full time on the cause and to extend the reach of their advocacy, and that many of those donations come from individuals or organizations who stand to benefit if some of the think tank's desired policies are implemented, does not make them "legalized corruption".
[1] http://www.irs.gov/Charities-%26-Non-Profits/Lobbying