Because of course he is. The tar sands must flow, and if they flow across and/or into water supplies and aquifers, well, that's all part of Making America Oily Again.
Trump once had a financial stake in DAPL[1], but it seems he sold off his stock in the various companies involved., Trump has a stake in DAPL, at least, beyond just placating Big Oil, et al. Now it's just about pumping oil as fast as it can be pumped, and never worry about the results.
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Trump to give green light to Dakota Access, Keystone XL oil pipelines
President is expected to issue executive orders to move the projects forward.