For one thing, usually you get paid after you finish a job, so if you said “its just an unpaid demo”, theres be no evidence of your lying or not. “This foreigner is just doing a demo” is a classic line by cram schools that are getting busted for long-term illegal employment of foreigners that dont have work permits. There are too many other discussions on this subject on this site for me to go into further detail.
But actually, im pretty sure that any work, paid or not, requires a work permit. So that time I grabbed some trash bags with some friends and we cleaned up the beach (before I had an APRC) was “working without a permit” and therefore technically illegal. Obviously immigration has better things to do, but if we’d posted all over social media about it and someone was having a bad day, we could have gotten in huge trouble (based on “the letter of the law”)