If I had to take a guess, I'd say this has something to do with it:
"Aggressive mode is faster than Main Mode but not as secure. However, aggressive mode uses half the number of messages, has less negotiation power, and does not provide identity protection."
Astaro is a security product after all, so it would make sense not to support unsecure methods.
Aggressive mode is insecure... thus they dropped support for it. I think even Cisco has changed the defaults on newer builds of their security IOS, etc. to default to Main Mode.
Probably a leftover from the OEM mfg. they didn't turn off... Astaro OEMs that client from another software vendor. Also, that client can be used to connect to other IPSEC VPN concentrators, not just an Astaro, so they may have left that option in purposefully (though I wouldn't, given Main Mode is safer and supported on most / all IPSEC VPN concentrators).
