I dunno about ostensibly, that is really the reason.  Morocco and the Sahrawis are not members of any of the same multilateral organizations -- Morocco is in the Arab League and the SADR is not, the Sahrawis are in the AU and Morocco is not, Morocco is in the UN and the Sahrawis are not, etc.

It led to a fair amount of diplomatic tension between South Africa and Morocco when the Pan-African Parliament became based in South Africa; because the SADR is a member of the AU and has representatives in the PAP, South Africa felt that it finally had to carry through on a longstanding promise to formally diplomatically recognize the SADR.  Mandela had made this promise during the anti-apartheid struggle, when the ANC allied itself with the Polisario.  (The ANC was allied with many groups involved in armed struggle; the thinking at the time was that the ANC couldn't afford to be choosy about its friends, and needed solidarity with other groups it considered fellow liberation movements, whether they were ideologically aligned or not.  These included the PLO, the IRA and the Tamil Tigers, all of whom the ANC-led government retains some degree of ties with to this day.)  So anyway, it was a nice enough promise when the ANC was in exile or opposition, but it became diplomatically inconvenient once the party came to power, so the SA gov't dragged its feet on recognizing the SADR because they didn't want to piss of Morocco.  But when the PAP came to town, they had to pony up.  (The alternative would have been allowing the Pan-African Parliament to be based in Libya, which doesn't actually have elections or any form of representative democracy, not even a flawed one....)

/tangent