The criteria attribute of the  FHIR R4 Subscription resource id defined as a FHIR search criteria without the base url.  The FHIR specification defines search criteria as follows:

• A specified resource type: GET [base]/[type]?parameter(s)
• A specified compartment, perhaps with a specified resource type in that compartment: GET [base]/Patient/[id]/[type]?parameter(s)
• All resource types: GET [base]?parameter(s) (parameters common to all types). If the _type parameter is included, all other search parameters SHALL be common to all provided types. If _type is not included, all parameters SHALL be common to all resource types.

parameters(s) are defined as the following "the parameters are a series of name=[value] pairs"

The MedMorph profile specifies Subscription criteria to take the form of "[ResourceType]?all". 

"?all" does not follow the search specification for parameters being named value pairs.  Furthermore the concept behind ?all is superfluous as simply specifying the criteria as   [ResourceType] would achieve the same result and fit within the FHIR search criteria specification.