While it's shitty to RSVP for something like a wedding and then not show up without a good reason, and also poor planning to RSVP for something when you might be able to foresee conflicting obligations, playing in a fairly big match seems like it might qualify as a good reason and be something the couple should forgive of their friends. 15-20 people out of 130 not showing up shouldn't exactly ruin the wedding.
Now, if the couple is out additional costs that would have been borne by these gueststhe cost for a room?then I'd say it's mostly on the guests that RSVP'ed that then didn't show to compensate the couple, either by paying the cost for the room anyway, or at least giving an extra-generous wedding gift. I wouldn't say the venue is really obligated to do anything, given that they're just sticking to the agreed-upon terms. (It would be nice of them give a discount, of course, but I wouldn't say they're obligated or assholes if they don't or anything.)
(If the couple is complaining about costs for things like food or drink that will now simply go unconsumed, then I'd say they should consider those sunk costs since they're paying the same regardless of who attends. Again, maybe a justification for a extra-generous gift from the no-shows.)