If you're talking about Jabari Bird's deal, he was a super late pick in the draft, where 99% of the dudes wound up getting cut anyways. Rather than give the Celts a bigger target to cut (i.e. take a "guaranteed" contract where he could get cut in the case of a signing, etc.), he and his agent wound up taking the 'easier' route wherein there would be less money right off but a lesser chance to get cut with the opportunity to learn under the best player development department in the league.
Contracts are a two-way thing. Just because you can do something doesn't mean you can.