The Beraisa is relying on the previous page's analysis that the Halachah of workers applies to all types of fields. The Beraisa here is specifically discussing the laws of Ma'aser and not those of a worker, and is teaching that a worker need not worry about Ma'aser while in the field, just as the owner can eat there freely. The Gemara explains this to refer to a fig tree on the border of the owner's property because, otherwise, what is the Chidush here? It is obvious that any person (even a buyer) is exempt in the field before it enters the domain of the owner! Therefore, it must be teaching a difference regarding the domain itself, whereby the owner would be exempt but a buyer would be obligated to take Ma'aser. This is an acceptable interpretation of the Beraisa, which is not dealing with the vineyard but with the laws of Ma'aser derived from that case.

Yoel Domb