What Acrobat/Reader allows depends on the document. For example, If the document has any of the following characteristics, you won't be allowed to e-sign, but you may be able to digitally sign:
1. The document is Reader-enabled
2. The document has any digital signature fields. This includes a certfied document.
3. The document includes a button with a "Submit a form" action.
4. Other things.
If you can provide a link to a sample that doesn't allow this, we could take a look and tell you the specific reason.