There is no STEM shortage; tons and tons of STEM workers work outside their field because they can't get a (typically very competitive) job in their field. If there were a STEM shortage, STEM wages would be skyrocketing, which they are not. Many people with PhDs in the sciences are underemployed and underpaid.
More STEM workers means lower pressure for wages to rise, thus saving money for companies in addition to allowing them to be more picky about who they hire. Increasing the pool of STEM workers is an easy way to please the tech companies (who appear to be coming into their own as political juggernauts) and seeming to make progress on "immigration reform" while circumspectly fucking over the employees of the most advanced sector of the economy in the name of progress.