Yeah, but notice that most volunteer work has a special feel-good element to it? Feeding the homeless. Saving lost animals. Helping disadvantaged children.
There aren't many volunteer organizations that, to borrow dgallagher's example, mow lawns or rake leaves, and the ones that do, only do it for special places. Perhaps for a monument or a park.
Obviously people would mow lawns but the price would be higher because they don't have to. If you wanted someone to mow your lawn, you would probably need extra income. Or, since you have more free time, you can now mow it yourself and enjoy it instead of feeling like it's cutting into your only spare 48 hours.
The attractive part of basic income for me is that everyone has enough to buy food and housing, so suddenly menial laborers have an iota of bargaining power.
There aren't many volunteer organizations that, to borrow dgallagher's example, mow lawns or rake leaves, and the ones that do, only do it for special places. Perhaps for a monument or a park.