If people willingly choose to be unemployed for $15,000/yr (as an example income), that's fine. There's no reason we need to be at 100% employment. Some amount of the population just doesn't need to work.
The problem is, basic income will likely lead to inflation. The government would need to control prices in the necessities market (food, rent, utilities, etc) to make basic survival possible on $15,000. The price of luxuries can go up as long as necessities are affordable.
There's your incentive. You can survive on the basic income, but if you want any luxuries, you have to have a job. That way the people who don't want to contribute to society are taken care of, and there's an incentive to actually get a job paying more than $15,000. I'm sure we've all heard stories about families on welfare buying expensive toys, TVs, Xbox, dirtbikes, cable TV, etc. If we want to get rid of that, welfare needs to pay exactly what it costs to survive, and nothing more.