That's because the package is called `camlbz2` in Opam, but the actual OCaml/`ocamlfind` package name is `bz2`, hence e.g. in `_tags`: 

```
$ grep bz2 _tags
true: package(bz2)
```

Also:

```
$ ls ~/.opam/4.01.0/lib/camlbz2
$ ls ~/.opam/4.01.0/lib/bz2
bz2.a  bz2.cma  bz2.cmi  bz2.cmx  bz2.cmxa  bz2.mli  libcamlbz2.a  META
```