As a general rule, everyone can seat anywhere they want in any bus in Israel. There are however, a limited number of bus lines that run strictly inside orthodox neighborhoods (mainly in Jerusalem) where there are some other rules that may be in effect and this may be one of them.
There is a good explanation for this in
Wikipedia:
Haredi bus lines
Some lines, mainly running in and/or between major Haredi Jewish population centers, are classified as 'Mehadrin' buses. These buses, while identical to others, can be used by anyone whether Haredi or not, but travellers should note that men and women (with the exception of husband and wife, or parents with children) are not supposed to sit next to each other, and women may be expected to sit in the back of the bus, while the men are supposed to sit in front. For women, a modest style of dress would also be recommended (meaning, no miniskirts or bare shoulders). These are guidelines that most of the riders of these specific lines insist on. These rules on these mehadrin buses are not 'law', and anyone can ignore them, but it is seen as disrespecting the local population