The theory was that 529 plans are only useful to affluent savers. Thus, if taxed, he can generate revenue from those affluent savers in order to provide a tax credit to lower-income (than the affluents) families. The problem is, affluent savers aren't the only ones who benefit from these plans. Not everybody has a quarter million dollars in their 529 accounts like the Obamas do, but many middle class, median-income families also use 529s to help ease the college burden.
The BLUF here, in my opinion, is that Obama thinks "middle class" only applies to people making at or below the median household income. He's playing on this slippery slope of including everyone above the 50% line (not absolutely, but somewhere in the vicinity) as "rich". I'm well above that 50% line, but I ain't rich. If I were really rich, I wouldn't do this job, that's for sure.
Obama wants to assault wealth accumulation to provide income for lower income families. You'll forgive me if I'm not opening my wallet just yet.