That means you can just unstrap as a free action
Attacking with the shield is still a standard action, or part of your full-attack action, but not a free action.
Due to the amount of threads about the subject, even if their FAQ click count is considerable low, the development team just issue a FAQ about this item:
The throwing shield says that it has special straps “that allow you to unclasp and throw it as a free action.” It seems likely that “unclasp and throw” means “unclasp in order to throw” but it could also mean “unclasp and additionally throw” which could give a character any number of extra attacks. Which interpretation is correct?
Throwing shield’s wording means you can unclasp as a free action in order to throw it; throwing it would requires its own action. The wording will be updated to disambiguate in the next errata.
However, should the doubt arise again, there are FAQs upon FAQs about gaining attacks, or by ignoring other rules in very specific conditions, and none are actually allowed by the rules.
Your number of attacks is limited by general rules, like high BAB, number of natural weapons, abilities that call out that you gain an extra attack, or by using two weapons. Specific items or spells cannot ignore those rules unless it explicitly mentions doing so.
For example, the fact that you pick a race with four arms, capable of wielding four weapons, does not mean you gain extra attacks with them, unless the race also has an ability declaring that you can use the extra arms for extra attacks. Or gaining claws on all of your limbs (normally four), but the ability says you only gain two claw attacks.
This is also true for monks, even if they can use all their limbs, and some races could possibly have more than four limbs, on a flurry of blows, the number of attacks they can do is limited by their BAB/monk levels.
To simplify what the (older) FAQs say, normally you cannot gain more attacks per round unless it says you gain additional attacks.