It is a market distortion. It favors the industry that is getting government support, taking away valuable resources from other industries that would do better in a free market.
Practically, HN is pro-engineering, so we likely approve of such moves. Similar subsidies in other industries might not be viewed as favourably.
Personally, I am not against it, because as someone who has many of his peers playing a zero-sum game in mathematical finance, I've seen some of the downsides the top talent being allocated where the free market desires them most.
It's not just a pro- versus anti-engineering issue. A lot of engineering fields are rigorous and difficult while not being very highly paid.[1] Part of the reason is the market distortion you point out: both on the demand side (e.g. aerospace is a heavily regulated industry with few opportunities to build "unicorns"), and on the supply side (e.g. the government invested heavily in churning out aerospace degrees during the Cold War).
[1] I made more money out of school in software, with no relevant degree, than I would have in aerospace, with a B.S. in the field.
The Econ 101 answer is that we'll have some number of people going into STEM fields would be more productive to the economy (and probably personally better off) as lawyers or cops or bakers or whatever. More realistically, it's likely to cause average salaries for engineers to decline somewhat as supply catches up to demand.
Not just the tech industry too. There are plenty of areas of STEM which are totally saturated with smart effective highly educated people and the result is low wages and poor employment outcomes. Just look at chemistry or biology.
I worry more about quality going down than either of those. Companies will amass huge armies of truly awful engineers and it will become a profession of attrition.
Not to mention that we will motivate foreign students to study mostly only STEM and thus deprive ourselves of the diversity of thought and the artistic and critical labor output of foreign workers in non-STEM domains.
Makes me think of Arcade Fire: "One day they will see it's long gone."
The tech industry isn't zero-sum though, even from just a purely selfish "can I get a job" POV. More talent in the pool will arguably make the industry bigger, rather than diluting the value of that talent. There's just so much room to grow.
But fewer complaints of H1-B wage slavery (note: I'm exaggerating somewhat), which also suppresses salaries. Hard to say which is more significant an effect.
It's government trying to pick winners, which rarely works out that well. I think government should stick to basics like infrastructure, education and making sure everyone has decent health care; that kind of thing that companies are less suited for.