Your logic is flawed, because

weak field = few world ranking points.

Westwood for example has to finish Top 3 at least to not loose any points so his average won't drop.
Its a tough task but he looks to be on course.

Its not to accumulate ranking points, otherwise they would avoid these events and only play in the strong ones where a weak finish would still give them points.
Rory is obviously playing to sort his swing out.
Lee is playing in Sweden for appearance fees.

and tiger is playing the greenbriar.  Also not a stellar field

As we've seen on PGA and other tours, it's not just the top ranked players that are capable of winning.

Besides, we can be critical of many tournament fields, even the ones Tiger plays in. I'm sure the sponsors and the local fans are proud of their tournament and thrilled to get any big names.