I came across this mangling of the standard “as they are wont to do” in an award-winning book published by a mainstream publisher and written by a Pulitzer Prize-winning author.  (On the same page “pretension” came out “pretention.”)  I don’t recall encountering this usage before, but a Google search turns up 382,000 of ’em.  Plus 2.6 million occurrences of its cousin “as is their want”–this one actually outpolls “as is their wont,” which garners only 558,000 usages. (Google helpfully asks me if it didn’t really mean to search for “as is their want.”)  So this is an idiom that seems to be having an identity crisis.  I have a pit in my stomach just thinking about it.