None of what you link shows anything about SAG demanding actors to do unpaid work. The second option specifically states that you must provide vouchers and then ulimately proof of paid work on three separate occasions as a background actor. The first option is a no-brainer in that you're getting paid as a lead. The 3rd option of being a member in good standing with another union also has nothing to do with doing unpaid work.

in practice, this all means: to be able to apply, you have to have worked 3+ times as non union. Which is usually unpaid or paid extremely low.

This is even worse on the side-unions, such as costume, etc.

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?