If he were to veto it, it would already pass again in the House with a 2/3's vote. The Senate would require some politicking, but it's not unimaginable that it could also get a 2/3's vote.

Having your veto overturned is a bad thing. That alone would be a reason. The other reason being that Obama chose to stay silent on the SOPA/PIPA/CISPA issue the first time around. Coming out against it would have cost him relatively little in public opinion, which must mean there are interests relevant to him that aren't entirely opposed to the bill.