Same happens when you go to Japan,Australia…as. volunteer. You always says that you are visiting as a tourist (totally true) staying with friends (housesit) or i preffer to book something in booking.com for the first night (or just booking one tht you can cancel) and show it to them. In some countries to volunteer you need to have a work visa even it’s for free. so, just tell them about your tourist plans
These are random checks. You should still be prepared for questions or interrogation in case.
I’ve already been to the US and had to answer several personal questions about money, work, family….without being isolated in an interrogation room. It wasn’t for a warm conversation. It’s a difficult experience when you’re traveling alone. Solo travelers are more likely to be suspected.
It’s also profiling. Young travellers will be more likely to be working illegally than people who look like retirees. Different nationalities will be selected in some countries as they are more likely to overstay visas. A tourist visa is just that, purely for tourism.
I did a long trip to the USA in 2017 and was interrogated by 2 different people. The second one was friendlier and admitted me in the end, however, it was not a pleasant experience by any means. If I go there again in future, I might just go for a couple of weeks to avoid all the hassle, and don’t think I would dare try a pet sit there now.