The WTA, imo, applied the rules set down by the ITF TADP correctly. Those rules just have a gaping loophole in them:
http://www.itftennis.com/antidoping/rules/tadp-overview.aspx
10.11 Status During Ineligibility:
10.11.1 Prohibition Against Participation During Ineligibility:
(a) No Participant who has been declared Ineligible may, during the period of Ineligibility, play, coach or otherwise participate in any capacity in (or, if the Person is a Player Support Personnel, assist any Player playing, coaching or otherwise participating in any capacity in):
(i) any Covered Event;
(ii) any other Event or Competition or activity (other than authorised anti-doping education or rehabilitation programmes) authorised, organised or sanctioned by the ITF, the ATP, the WTA, any National Association or member of a National Association, or any Signatory, Signatory's member organisation, or club or member organisation of that Signatory's member organisation;
(iii) any Event or Competition authorised or organised by any professional league or any international or national-level Event or Competition organisation; or
(iv) any elite or national-level sporting activity funded by a governmental agency.
and
10.11.2 Without prejudice to the generality of Article 10.11.1, a Participant shall not, during any period of Ineligibility, be given accreditation for, or otherwise granted access to, any Covered Event or any other Event or Competition or activity authorised, organised or sanctioned by the ITF, the ATP, the WTA, any National Association or member of a National Association, and any such accreditation previously issued shall be withdrawn.
The WTA lawyers decided (and I think I would agree in terms of legal analysis) that just being in the draw is not "participating" in the event. For me "participating" start once one actually steps out on the court to play or does a promotional appearance (such as being at the draw ceremony).
The annoyance for Maria, though, is 10.11.2 specifically restricts them from granting her a credential or access to the tournament, causing the weird situation where she can't pick up her badge until the morning she plays (which is her first day being eligible again). Otherwise, I think you could make a viable argument that she could even be on site & practice there without "participating" in the tournament, although the latter would depend on how/if the tournament publicized/promoted her practice session- if they turned it into an attraction/encouraged people to go there, I would argue that would be her "participating" in the event.
The other interesting thing from this digging is the 10.11.1 (a) (iii) prohibition from "any Event or Competition authorised or organised by any professional league or any international or national-level Event or Competition organisation". If I recall she did a World Team Tennis hit n giggle last year. I think you could feasibly argue this was an "Event organised by a professional league", and therefore she violated the terms of her ineligibility. It's essentially a moot point though, no one in the ITF is going to try to take her on again after all of the bad publicity they got last time around.