How they do it!!! Lowes frequently offers FREE or cheap carpet installation.
It's a GIMMIC copied from Home Depot back in 2006. ALL carpet prices and pad prices were raised prior to the start of this promotion which continues to this day. One carpet went from $1.99sf to $2.99 (most others not quite so much). Basic labor was $75 cents sf to include install, take up & haul away, move basic furniture.
So for 1000 sf the labor would have been $750 but now it's either FREE with Stainmaster or cheap with other carpets. But if you bought the $2.99 carpet indicated you were paying 1000sf x $1.00 or $1,000 more for the carpet which more than paid for the so-called cheap install. PLUS the price of pad went up too. End result Lowes makes more from a FREE or cheap install than they did before.
Beware of cheap labor prices and only look at the TOTAL install price before making an educated decision. This practice also applies to Home Depot.