The reason one must be careful in what one says at the border is that regardless of what visa or other permission one has received prior to travelling to the country, the immigration officer must make a determination of the traveler's intent for entering the country. Hence the variety of questions. If the officer believes the intent is other than what was proclaimed in obtaining the visa then the officer may deny entry.

Claiming to be a tourist but giving information that can be interpreted as a reason for conducting business or possibly a compelling reason to overstay is reason to deny entry.

Don't be evasive but don't give more information than is required while at the same time be truthful/accurate.