The reason billions and billions are being repatriated is because those earnings were left offshore (with a very low effective tax rate attached) and, absent an amnesty like this or a dire need at the US level, would never have come back onshore to be taxed. Taking 5.25% of something is better than 35% of nothing.Well, no actually.
According to the Congressional Budget Office analysis of the American Jobs Creation Act of 2004 (here), the particular provision in question, "increas[es] receipts in 2005 by about $2.8 billion and reduc[es] receipts thereafter." In other words, the net effect of the provision, over time, is to decrease revenues, since offshore profits do ultimately get repatriated.
The analysis does not break out the precise amount by which receipts are decreased by this one particular provision. However, this provision of the Act, together with the three other provisions in the international tax area, were expected to "increase revenues by about $1.3 billion in 2005, and reduce revenues by about $12.1 billion over the 2005-2009 period and $42.6 billion over the 2005-2014 period."