Answer

The Talmud there brings other laws like this as well. Both in the Talmud and in Maimonides it appears that this is a special, specific ruling and not a general law. In these particular cases, the Sages did not enact the prohibition. And in fact it requires further investigation what is unique about them. Perhaps there is a general principle there of performing labor in an unusual manner, such that specifically regarding this kind of rabbinic prohibition they did not enact a decree against it (perhaps because when one does it in an unusual way, that itself reminds him that today is the Sabbath, and he will not come to a more severe prohibition).
In the halakhic authorities it appears that according to all opinions, a rabbinic prohibition done in an unusual manner is permitted (a double rabbinic prohibition). However, here it is not clear whether in a case like this of sucking, it is a double rabbinic prohibition or only a single rabbinic prohibition.