Are you sure it is really a Bing bot? I have seen bots that ignore disallows that turned out to be from somewhere other than expected when doing a reverse IP lookup using dig.
dig -x 126.96.36.199
; <<>> DiG 9.9.5-3-Ubuntu <<>> -x 188.8.131.52
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 30244
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 512
;; QUESTION SECTION:
;184.108.40.206.in-addr.arpa. IN PTR
;; ANSWER SECTION:
220.127.116.11.in-addr.arpa. 71 IN PTR msnbot-157-55-39-142.search.msn.com.
;; Query time: 57 msec
;; SERVER: 18.104.22.168#53(22.214.171.124)
;; WHEN: Thu Jun 12 18:18:06 EDT 2014
;; MSG SIZE rcvd: 104
The reverse lookup shows the IP is indeed related to Bing's search bots.
They do state that the crawl control directive in a robots.txt file will take precedence over a configured setting on their end.
If that doesn't seem to be the case you may want to try using their controls to adjust their crawl for your site.