Dear Itay,
We have received the answer from PayPal Merchant Technical Support team. They said,
"Unfortunately, taxes from the profile is not supported when using the subscription buttons, nor can you pass the tax variable in subscription code. This is a limitation of the subscription buttons you can not have taxes or shipping."
So, that is the reason why sales tax was not being calculated. We will remove the sales tax property from the subscription button so that it wont raise any more confusion.
Please do not hesitate to write us back if you need any more assistance.