Ah. We delve into the mysteries of the IRS.
Because if you COULD take the deduction, and decide not to, the IRS will "impute" it -- that is, determine that you should have taken it and did not, therefore you lose the tax break of claiming the depreciation deduction, but gain all the liabilities of having claimed it!!! A lose-lose for the taxpayer, and a win-win for the IRS!!!
CAVEAT: This is NOT tax advice!!!!