Why would I view a man who sought an Empire for Slavery by launching a vicious imperialist war against Mexico to introduce slavery, a practice unknown in post-independence Mexico, to a vast swathe of land in a favorable light? 
Because Polk's expansionism was not motivated primarily by a desire to expand slavery like the later Southern "Fire-Eaters" wanted and none of the territory annexed in the Mexican-American War with the exception of some areas gained by Texas ever became a slave state. Polk while a Southerner and a slave-owner was not ideologically obsessed with keeping and expanding the institution of slavery. Instead his desire for expansionism was primarily strategic with a view to providing the Republic with a window to the Pacific and preventing European powers from gaining control over areas such as California due to the weakness of the Mexican government.
The war was provoked by Tyler Sec of State John Calhoun, slavery's biggest proponent, admitting Texas to the Union, and most of the pro-war advocates in Congress were Southerners looking to secure that empire for slavery. The Mexican Cession only didn't end up as slave states because, other than California, none of it was admitted to the Union before the Civil War. Polk (and Tyler before him) provoked an utterly unnecessary war with Mexico in hopes of stealing a third of Mexico. How is this something to celebrate?
The war started over a year after Texas annexation and Calhoun was opposed to the Mexican War for the very reason that slavery might be excluded from the annexed territories. Plus considering the Republic of Texas desired annexation, I don't see any objection for the United States to annex it.
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