In practice there may be shifty individuals specifically exploiting the situation - but it sounds like SAG requires that it members be well compensated and treated fairly and tries to make it easy to approach joining the union from a number of different angles.

If there's exploitation going on here I don't think it's really fair to attribute it to SAG in particular (though I'm not certain if you ever were, your original comment just mentioned the exploitation of hollywood - not specifically that SAG was driving this). Maybe it's more the case that a landscape that's likely to be rife with exploitation (and actors and abstract artists are extremely vulnerable to exploitation as a class of employee due to the strange quality of the value they produce) is actually made a little bit better by SAG ensuring that most actors are getting a fair shake?