Charging their laptop at a library intended for use by the public (which they are a part of) burns less energy than someone who leaves their air conditioner running while they go get groceries.
Most homeless people go out of their way to not waste public resources and services because they are used to criticisms like this, usually issued from a vantage point of indifference and ignorance.
I believe the actual resource being referred to is the aforementioned human capital. The article mentioned that he was a network engineer, a career path that can easily command $60K/year on the low end.
Network engineers that show up to work and crank out widgets don't get to stay in that position. Those that seek to learn and grow, while reliably performing there jobs are typically the last on the chopping block and if they do get RIF-ed, have a strong market to fall back on. This is a good use of his time. Figuring out ways to mindlessly click/watch garbage resources on the Internet to scrape up an occasional pizza is laughable, when compared to his potential. Based on the information presented in the form it was, this looks more like a failure of the person than the system.
Speaking of information presented, I see no reason to think you have any more insight into a homeless person's motivation than you do for passive-aggressively insinuating the poster you are responding to is indifferent and ignorant.
They are doing one of the least productive things I can imagine. They are actually wasting public electricity and services.
It's fine if they are mentally ill, or disabled, but I doubt that's the case.