So are we talking about open jam sessions and not a rehearsed concert?
I think that there is a huge difference between the two. If you know up front what you will play, you'll know that you can probably make it.
At a jam session, you're not really in control and if you don't know a certain tune, you might "butcher" it. Same thing would happen to Jazz drummers if they had to sit in at a Prog session and play an unfamiliar Rush song with signature grooves and fills.
Even if an unknown tune is not very fast and has a standard form like 32 bars of AABA, there are some dangers. For instance, if you don't know Nardis and you just play spang-a-lang with sticks from beginning to end, it won't be a lot of fun.
Anyway, you can make it easier for yourself if you attend a regular jam session as a listener and find out which tunes get played often. And of course you could ask which tunes the other musicians want to play before you sit in (or propose one yourself).
Luckily, some of the stressful factors of joining a session don't apply as much to drummers. If you don't know the changes, you're out of luck as a bassist (unless you have really great ears and are flexible). As a drummer it's easier to play an unfamiliar tune if you pay attention to the other musicians.
more open jam sessions, but also try out situations, or fill in situations.
and it is not the majority of situations, but at least around here, it is something that has always happened. There are also a few leaders and players who have a rep of being harsh just to be harsh...no one plays, or gets jazz like they do for whatever reason
and it is not just the jazz scene either...it happens on the orchestral scene too...lots of "looking down noses"
I used to let it really get to me util I heard other drummers - who I KNOW were really good - complain about the same people/situations