Check out this youtube link where Sylvia of WP makes a speech.
Between 3:55 to 4:20 states that a by-election is legislated by the
Parliamentary Elections Act.
Based on the video, in 1999, when a member of Jalan Besar GRC
was convicted of criminal charges and forced to vacate his seat,
Wong Kan Seng, then House Speaker, gave the following reason for
why one member's departure should not trigger a by-election in a
GRC:
"The legislative intent was not to allow any particular MP
so-elected as part a group of MPs in a GRC to hold the group to
ransom through resignation or otherwise. The thinking then was that
all the others MPs should not be forced to vacate their seats on
account of one MP vacating his seat for whatever reason; death,
resignation or whatever."
http://theonlinecitizen.com/2008/08/workers-party-rejects-motion-to-amend-parliamentary-elections-act/
wow, who can hold the pap to ransom? not even the president,
remember ong teng cheong!