If the TS is planning to sell to a US Stock Agency, he will have no choice commercially but to comply with their terms. This is similar to the situation in Singapore where if you want to apply for a credit card, you have no choice but to agree to their terms. Ie its operated on their standard terms.
For discussion, if for example, a US agency buys the photos from you, and does not require the model release. What is the exposure?
The model in Singapore, if she wants to take action, would have to go to the United States to sue the US Agency. The model would also have to show that US law applies to her cause of action (which is questionable since the model was engaged in Singapore, the shoot done in Singapore etc). I have not checked the applicability of the US law viz creation of images in other jurisdictions, but as you can see, it is not a simple matter.
It would already be costly to try to initiate litigation in Singapore, and this is multiplied in the case of cross border litigation. Unless your image is worth millions of dollars and the model is some super celeb with deep pockets, the chance would be remote.
Of course however since most US agencies will not accept submission without a release, this issue is rendered commercially moot.
That may be so for other countries (which I cannot comment since I don't know), but in Singapore, until case law decides otherwise, you can use photos taken without a model release, for any commerical purpose, and not limited only to editorial usage.
The reason I ask is coz I'm considering selling to stock photography website, and also, a local potential client would like us to shoot those 'street photography' - people should be "incidental" - meaning, really there, doing things in the foreground, and not models in posed pictures. I'm just wondering whether these people need to sign release lor. The use is for website only.
So... don't need a release if for Singapore website?