Once a contributor uploads a package to the AUR they are considered the contributor. If they made a mistake it is acceptable to replace the package with a new one, or request that it's just removed from the AUR.
The act of uploading the package can be considered a donation or patch submittal. Everyone acknowledges that the package was created by the contributor, but it is now the property Arch Linux.
Because the developers are acting for the greater good of Arch, they are allowed to change the packages as they see fit, before adding them to the tree that they belong in (official, unofficial, etc). The contributors have no say in this matter as it is no longer their package.
If the developer is confused about something, they can consult the contributor for suggestions/clarification. This is not required, and should not be expected. The developer is just as justified in not consulting the contributor and instead asking someone else who can help.
If a contributor has a problem with the final product in the tree they bring it through the regular communication channels: mailing list, forums, IRC, etc.. The contributor should be careful not to direct any issues to a particular developer, especially if they are just going off of who committed the package (a package could have been worked on by three people and then committed by one of them). All developers are equally "to blame" for any "mistakes" in the packages, if you want to look at it like that. That way there are no personal attacks and no one "makes all the mistakes". Instead we work more as one whole and we all accept responsibility for each other's actions (we trust each other, right? That's why we were asked to become developers... someone trusts us...).
The final document should make it very clear that the packages that are uploaded are now the property of Arch Linux and do not reflect on the contributor; the contributor is recognized however as the person who created the original package.