I had the unfortunate need to understand the licensing model a little bit better recently for enabling editing in Office Web Apps.  It's a little bit of a circuitous route so I thought I would just quickly detail it here.  In short you want to:

  1. Create a new license mapping for folks that are going to be editing Office Web Apps.  This license mapping is created with the New-SPUserLicenseMapping cmdlet, and it allows you to say "this claim" maps to "this right".  The list of rights is hard-coded, and you can get them with the Get-SPUserLicense cmdlet.  For editing Office Web Apps you want to use the OfficeWebAppsEdit right.  The "this claim" can be an Active Directory group or any other claim that your users are going to have. After you've created the mapping you want to add the mapping and turn on licensing.  For more details see http://technet.microsoft.com/en-us/library/jj219627.
  2. When you create the Office Web Apps farm use the -EditingEnabled switch; you can also use this with the Set-OfficeWebAppsFarm cmdlet after the fact.  For more details see http://technet.microsoft.com/en-us/library/jj219436.

So a complete PowerShell script for this would look something like this (assuming I'm using membership in an AD group called "OWA Editors" as the claim required to edit):

#NOTE:  this is using an AD security group so I'm using the -SecurityGroup parameter
#if I was using FBA, then instead I would use the -Role and -RoleProvider parameters
#if I was using SAML, then instead I would use the -ClaimType, -OriginalProvider and
#-Value parameters, or you can just use the -Claim with an SPClaim parameter
$a = New-SPUserLicenseMapping -SecurityGroup "OWA Editors" –License OfficeWebAppsEdit
$a | Add-SPUserLicenseMapping
Enable-SPUserLicensing
New-OfficeWebAppsFarm -Verbose -InternalUrl https://<machinename> -ExternalUrl https://<fully.qualified.machine.name> -CertificateName <FriendlyNameOfCertificateFromPreviousStep> -ClipartEnabled -TranslationEnable -EditingEnabled

For more details on getting Office Web Apps set up in your farm you can also see my earlier post on it here:  http://blogs.technet.com/b/speschka/archive/2012/07/23/configuring-office-web-apps-in-sharepoint-2013.aspx.