What it says on the tin: because last time I checked, the Heart pirates were a crew too.
Long before the Dressrosa arc, Law and Bellamy stood in an interesting comparison of character: Law, once connected to Doflamingo, doesn't want to know. Bellamy on the other hand is trying very hard to return to the Doflamingo nest, despite getting pointedly rejected. Chances are, Bellamy's going to get neglected by Doflamingo again once he fails to win the tournament sooner or later. But what if he subsequently meets Law, and their common ground becomes apparent? The precedent has already been set in terms of a former captain joining Law's crew after all. Then again, the two might clash. Cue speculation.