I don't remember whether it was in a letter, but I do remember a meeting with elders & ministerial servants in which it was stated explicity that the reason was to avoid being taxed.
I was actually not at the meeting because I was in transit to some work (JW related) in another country. However, a ministerial servant in my home congregation called me right after the meeting and excitedly said "hey, the end is coming!" He thought that the fact that we might be taxed was some kind of indication that big things were about to happen.
The tax situation started with one of the big name televangelists and it was thought that it might affect JWs, too. I can't remember exactly who it was; maybe somebody else will remember. Was it Jimmy Swaggart?
Anyway, yes, it was for tax purposes that the donation arrangment was instiuted.