Most librarians are women and many make their way up the career ladder only to drop out/work part time when they have children. When they (and that includes me) return they may be grateful to have any job - particularly as most jobs seem to require knowledge of systems which may not have existed when they previously worked. If they are the second income earners in the family, level of pay may not be so important. Sometimes they trade income for job flexibility. However, it doesn't really help those who need to earn a decent income and devalues the real worth of information professionals.
I should also like to comment on the disgraceful pay scales for librarians (often clerical scales) operating at some of our best universities. They are always far lower than for IT staff (including non-graduate technicians) and comparable with office staff such as secretaries.