They don't, you're just obligated to report your global income as a US citizen. This isn't a big deal if you plan to never go back to the US to live there, but if you do (or aren't sure), this becomes a major issue because all those years you didn't file your taxes with the IRS makes you effectively a tax cheat whether or not you owe any taxes at all.
If you pay income tax to another foreign government (that has a tax treaty with the USA) and you earn below the threshold for an individual or a household, then you don't pay taxes at all. In order for the IRS to determine this they need to know how much you made. I don't know what the actual threshold is for individuals, but in the 5 years that I've been living outside of the US, I haven't paid a single cent in US income tax. It's a pain in the ass, but whatever, you get used to it.
If you just want the batter to run to all the bases and reach home and score a point (or a run, by baseball jargon) with no regard for on field ruling, then yes by a series of errors being committed by the fielders.
For example if a fielder throws to first to get an out, but throws it out of reach from any fielder. However, in this case the ball MUST NOT leave the field of play (e.g., get thrown into the stands), otherwise the ball is dead, and the runner will be placed on a base at the umpire's discretion.
This however is not a home run by rule, but rather a fielding error(s) that results in a run.
Another option is that none of the fielders try to get the ball. They just stand around after the batter bunts the ball. There's no real reason this would happen, unless something extraordinary happens (aliens land on the stands, that sort of thing). This would probably be ruled a home run, but again, astronomically unlikely.
If you're allowing swinging bunts, you could potentially have a batter "doink" the ball over the in fielders but outside the reach of an outfielder and have the fielder run past the ball without touching it (if they touch it, it's an error). If the batter runs fast enough as all of this happens you can conceivably have a home run this way.
The "I'm trying to break physics" version of this may have been covered by XKCD at some point.