You are correct about the V6 vs. 4-cylinder tow ratings. However, in this case, the hitches have the Class III/IV rating, not necessarily the RAV. But it's the same hitch hardware, nonetheless. $150 (including shipping) for a Class III/IV hitch is great, considering your shadow mount was $120.
To date, I haven't seen a Class I or II hitch made for the 2006 and the aftermarket manufacturers may take some time deciding whether to make them. Their take might be "Why design and make two different rated hitch receivers when you can get by with one?" All depends on how many they'd sell, I suppose.
If there's a need for a tongue (shadow mount) or 1-1/4" hitch receiver with the 2006 RAV4, I'm sure someone will manufacture one.
//Tony -- Three-time RAV'er!