By Ari Shapiro
The gemara says that we need both cases because if I only had 1 I would think that you take a shevua because you are moreh heter but the other one where there is no moreh heter you would not take a shevua, ka mashma lan the second case in the mishna that you do swear. The question is, why do you swear in the second case?
The Ran offers 2 pshatim.
1. Ka mashma lan that in both cases there is moreh heter. In other words the shevua is because you are moreh heter in both cases. The Ran points out that this is very difficult. If so, what is the gemara's question on Sumchus and what is the answer? Why didn't the gemara answer that the mishna could be like Sumchus and there is a big distinction between the 2 cases. In our mishna he is moreh heter and therefore he swears while in the case of Sumchus there is no Moreh heter and therefore there is no
shevua. Also, why does R' Yochanan have to come on to a new reason for the shevua (that we don;t want people attacking other people on the street), we already have a reason for the shevua in the mishna, moreh heter. Based on this he offers another pshat
2. Ka mashma lan that even where there is no moreh heter you swear because even though he is chashud to steal he is not chashud to lie with a shevua. This answers both questions. There is no simple distinction between our Mishna and Sumchus because even in our Mishna you swear even when there is no moreh heter. This also explains why R' Yochanan had to come up with a new reason for the shevua because until we haven't yet offered a reason for the shevua in the Mishna.