It'd take quite a while to type (again) all of the reasons why using the ISS as a sample return lab is a bad idea. The Reader's Digest condensed version includes:

1. The difficulty of placing the return sample into a compatible orbit where it can be retrieved and brought to the ISS. It's a lot harder than it sounds.

2. The complete lack of microgravity isolation labs necessary to handle the samples.

3. The problem with what to do with the ISS should it become contaiminated. It's far too massive to boost out of Earth's orbit or even boost it to a substancially higher orbit, so it'll come down someday.