It shouldn't be, but the standard is not clear.
The spec is very dodgy in that it acknowledges that every server needs to pick its own policies:
> Disposition of SPF fail messages is a matter of local policy.
On silently dropping email, the following is listed as a "consideration":
> Other dispositions such as "dropping" or deleting email after acceptance are inappropriate because they leave uncertainty and reduce the overall reliability and utility of email across the Internet.
There is no MUST (NOT) in the spec when it comes to silently dropping email.
The intent of the author of the spec is to always provide feedback, but it doesn't actually say that in cleae terms.
