Can someone tell me why the current ppp code does the following in ccp.c?

/* LM auth not supported for MPPE */
if (auth_done[f->unit] & (CHAP_MS_WITHPEER | CHAP_MS_PEER)) {
/* This might be noise */
if (go->mppe & MPPE_OPT_40) {
notice("Disabling 40-bit MPPE; MS-CHAP LM not supported");
go->mppe &= ~MPPE_OPT_40;
ccp_wantoptions[f->unit].mppe &= ~MPPE_OPT_40;

Why does pppd disable 40-bit encryption with CHAP-MS authentication?
I am trying to set up a PPTP tunnel to a Cisco PIX that only supports 40-bit
encryption. I don't remember this being a problem with earlier MPPE supporting