You're assuming UBI perfectly and completely eliminates reasons to join exploitative agreements. This is unlikely, even assuming your UBI has perfectly equitable execution, as there are many more reasons than money people get into such agreements. Most relevantly deceptive advertising, but also various forms of emotional manipulation, just not being very smart, etc. Those so vulnerable deserve at least some protection, and defending them holds the line both practically and morally for mistakes in UBI implementation, setting a clear standard for society, cutting down on people making predatory offers, and keeping you safe when you have an off day where you might make a bad decision yourself.

UBI is not sustainable. For something that is sustainable (and proven), look into Islam's Zakat.