Bais Shammai's shita about melacha being started on Erev Shabbos (Shabbos 17-19)

Beis Shammai's shita is a little bit hard to understand. In the mishna there seem to be two types of things that B"S says is asur to do on Erev Shabbos. The first is to put something in a kli like putting the flax in the oven. The second is to give things to a goy. I'm going to ignore the second one here. Why, according to Beis Shammai is it asur to put something in a kli on Erev Shabbos if it's going to complete on Shabbos?

It seems to be a machlokes Amoraim. Rav Yosef holds that it's because of shevisas keilim. B"S holds that there is an isur d'oraysa to let your keilim "do" melacha on Shabbos even if there isn't a real maaseh. However, Rabbah disagrees with that and holds that B"S only says that there is an isur of shevisas keilim if the kli is actually "doing" a maaseh. None of the cases of the mishna have a real maaseh. So then why does he say they are asur? Because of a gezeira. We don't want you to start them right before Shabbos because you might come to do it on Shabbos also. That fits in very well with the last case of the Mishna which says that B"S agrees by the grapes that there is no isur. That's because it's only asur d'rabonon to do it on Shabbos so the Rabbonon weren't gozeir on Erev Shabbos. How does Rabba understand that case? Why should there be no shevisas keilim d'oraysa? The gemara asks the question on 19a and says that it's only an isur d'rabonon so there is no gezeira.

Tosafos explains that the gemara is obviously working with R' Yosef's opinion because according to Rabba shevisas keilim is d'oraysa and not a gezeira.
The Ran says that this gemara is even working in Rabba and the gemara means that there is even though the kli doing the melacha is asur m'doraysa just like if you yourself did the melacha, they weren't gozer d'rabonons on keilim. He doesn't really explain why but I'd guess it's because there's no need. For a person we're afraid that if we let you do this then you might come to do something similar but we don't have that fear with keilim.
The Ritva though says it means that there is no isur and gzeira d'oraysa. He says that R' Yosef holds that according to B"S there is an isur d'oraysa of shevisas keilim so that you should rest and not come to do a melacha on your own. There aren't many gezeiros d'oraysa but according to this opinion this is one example.