Why do Double IPAs tend to be sweeter than regular IPAs? (I personally have in mind Ninkasi's Tricerahops and Saint Archer's Double IPA.)
I would say some are sweeter, some aren't. Brewers do kick up the mount of malt in IIPA's to get the ABV up there. This could lead to a sweeter taste depending on the malt bill and fermentation.
To be classified as a double IPA it must have an ABV >= 7.5%. That requires more malt/sugar. However, the perceived sweetness can be a factor from yeast used and the Bitterness Ratio: http://www.madalchemist.com/chart_bitterness_ratio.html
Yeast can just process a certain amount of sugar inside the mash and therefore in double IPA there can be more sugar left in the beer, depending on the yeast that is used.