Countries who have signed on for visa-free travel have as part of the agreement a number of expectations for travelers. You can be asked at the border for proof of:
Health Insurance or coverage.
Proof of sufficient funds for your stay
Evidence of where you will be staying while in the country
Evidence of the length of stay
Be able to state the purpose of your visit.
These requirements have been around for decades, and apply to just about any country you travel to. If you were to apply for a visa, the same requirements apply, just more proof needed.
Now will they really ask these questions? Likely not, especially for us older, white, US citizens. Be young, of color, looking like you might be going to stay, then yeah, you get asked. But most of the time, you might just be asked how long you are staying, maybe where you are going, and why you are here.