But why do the Church Fathers insist on this wrong information? It's not just a few ignorant Fathers either; look at that same page in Schürer and you will find a list of citations (p. 408 n. 24) that includes Origen, Jerome, Hippolytus, and Ps.-Tertullian, with the Greek and Latin text.
Some scholars think they confused the Sadducees with the Samaritans. This is the position of Le Moyne (p. 151).
But might a misreading of Josephus have been involved?
At Antiquities 13.297, Josephus says this:
What I would now explain is this, that the Pharisees have delivered to the people a great many observances by succession from their fathers, which are not written in the laws of Moses; and for that reason it is that the Sadducees reject them, and say that we are to esteem those observances to be obligatory which are in the written word, but are not to observe what are derived from the tradition of our forefathers. (Whiston, from PACE; go there for the Greek)Josephus is talking about what in rabbinic literature will be called the 'Torah in Writing' and the 'Torah in the Mouth,' or the written Torah (= the actual text of scripture) and the oral Torah (= the tradition of the elders, as it is called in the NT, Mark 7:3, etc.). Josephus means that the Sadducees do not accept the Pharisaic Oral Torah but only the written Torah.
But I wonder...
If a Church Father read ταῦτα τὸ Σαδδουκαίων γένος ἐκβάλλει, λέγον ἐκεῖνα δεῖν ἡγεῖσθαι νόμιμα τὰ γεγραμμένα, would he perhaps have seen νόμιμα and thought νόμος Μωϋσέως?
Maybe it wasn't this particular passage, or any passage in Josephus that gave rise to the patristic misunderstanding about the Sadducean acceptance of scripture. But it seems worth considering whether an early Church Father was reading a Greek discussion of the Sadducean acceptance of only 'written Torah' and interpreted this as saying that the Sadducees accepted only the Pentateuch.
The misunderstanding then became exegetically useful as some of the Fathers explained Jesus' response to the Sadducees--in which he quotes Exod 3 to prove the doctrine of the resurrection (Matt 22:29-32)--as a concession to their rejection of all scripture outside the Pentateuch (Jerome, Comm. in Matt. 22:31-32). In other words, Jesus argued from texts that they accepted and still showed their ignorance of the scriptures and the power of God.
UPDATE (23 Jan 2014): McDonald (pp. 138-42) shows me that Bruce (pp. 40-41) has already thought of this (and I bet Bruce wasn't the first). Also, McDonald himself is doubtful of this whole line of thought; he rather thinks that the Sadducees may have just accepted the Pentateuch, or at least that their scriptures were not identical to those of the Pharisees and Essenes, since they rejected the doctrine of the resurrection. I'm still going with what I wrote above, but that'll teach me to read a little more widely before I suggest an idea (that was already suggested decades ago) or talk about what scholars believe (without reading one of the most accessible accounts of the topic).