First, consider the arbitrageurs, the short-term stock traders who try to exploit imbalances in the market. The imbalance is in the details. The simplest answer is that the market still has doubts about whether the Justice Department will approve the deal. Not huge doubts, not debilitating doubts, but enough to discount SDLI's price significantly.
Several reasons suggest only the second makes sense.
First of all, JDSU's stock has already taken a 20 percent hammering with the announcement of the deal. A few mutual funds, now holding one big fiber-optics company instead of two smaller ones, may have sold stock to meet their own demands for diversification. All that suggests that JDSU is probably not a great candidate for shorting. SDLI, on the other hand, may be a stock worth buying. And even at its inflated price, history is on its side.