1. They hate to say no.
There is a small, but non-zero psychological effect of declining someone's invitation. It is in effect a rejection, and even if people *shouldn't* take it as such, the fact of the matter is most do, including the people doing the declining.
Also, it is common practice to base invite lists off prior event rosters, either automatically using FB's Create Repeat Event function (which only re-invites Goings,) or manually by transcribing the list of Goings and Maybes, and assuming the Declines and Not Respondeds aren't interested. Some may not want to be de facto removed from the "list" for FOMO.
It is also common practice to repurpose event pages for different events to reduce the overhead of creating a new page, re-inviting, and re-posting all the content. "No"s would not be notified of this, so some feel it is better to "Yes".
2. They want to help promulgate the event.
When you RSVP "Yes" FB tells your FBriends (except in a few specific circumstances.) Some people who understand this behavior of the system and its consequences for market penetration, willfully exploit it to help promote events by increasing eyeballs (exposure.)
3. They want to endorse the event, or the idea it stands for.
Some events, especially new or controversial or ones that are floundering in the promotion department, may need some help generating interest (which is different from exposure.) We are not all the rational independent thinkers we think we are, and the approval of your peers (word-of-mouth) or people you revere (celebrity endorsement) makes you accept and value something more. RSVPing "Yes" is in effect a way to recommend something, at a slightly lower commitment level than inviting people or sharing.
Sometimes an event isn't just an event. Sometimes an event is metonymic of an idea. And some people might want to recommend this idea even if they can't attend its totem event.
4. They want the event or the idea it stands for to endorse them.
Some people like to be thought of as being associated with certain activities, groups, and ideas. Whether it be delusional or not, whether it be or dishonest or not some people want to communicate a connection to concepts, be they Charity, the High Life, Avant-guardism, etc! These people would RSVP "Yes" to events typical of their pretense even if they couldn't actually attend.
5. They want to monitor the event.
RSVPing "Yes" means you get notified of changes and wall posts. Some people are curious about the worlds that certain events represent. The information contained in event pages--their description, images, wall posts, and attendees--offers an insight into these worlds, that the FB newsfeed system will bring to your electronic doorstep, for free, with one simple click of a Going button.
Alternatively, some people might keep track of an event because they do know what it's about and secretly want to see it to fail; or to know who goes to that sort of stuff to be able to judge them.
6. They don't want to deal with it right now
Digesting event descriptions--especially extraordinarily long florid sagas, in which the When Where Who & How Much is buried in a mess of gratuitous verbal onanism, deciding whether you want to go, and assessing whether you *can* go takes time and effort. Individually it may seem trivial, but if you are any kind of social netizen (or a hot chick) events gang up on you. Some might RSVP "Yes" as the safest default.
7. They want to thwart the NSA
Facebook and its customers (advertisers, goverments, Skynet) are compiling their own profile of you; directly from the information you gleefully submit, but also through pattern analyses derived from your activity. If you dutifully RSVP "Yes" to events you actually go to, this becomes in aggregation a fairly simple and accurate, not only record of where you've been, but by extrapolation a predictor of where you'll be and when. Depending on how sophisticated the algorithms are it may also also betray your spending patterns, your values, your fears and weaknesses... Some people don't like this idea, and though it may be somewhat futile, they deliberately introduce confounding data into the system. False RSVPs could be one way.
8. They want evidence that your FB RSVP means nothing!
There are still those who believe that if you say you are going on the Facebook, it means you're going in the real world. Some of these people might make conclusions and take actions based on this fallacy. Perhaps your boss sees that you RSVPed "Yes" to that Phish concert the day you called in sick and to keep her from firing you, you want to be able to point to the 4 events you "went to" on the same night on 3 different continents as you repeat after me:
