Assumption, to an extent. [A small extent. The larger extent is the malnutrition and the fact that Sebastian knows an uncomfortable deal of what growing up was like for Porthos. But let's focus on the smaller extent:]
If you are just arrived, chances are your priorities have hitherto been elsewhere than on food. Most people, upon arriving, also carry little coin on their person. [He gives a small shrug.] One could think of it as a paying-forward system.
[Implying that people offered Sebastian free food when he was new and he accepted it? Yup. That didn't happen, but you know...]