If for is really a while in disguise, maybe they allowed while to have a empty condition to create a simple transformation from for to while.

In other words, if

for (a; b; c) {
   d
}
[download]

is translated into

a;
while (b) {
   d
} continue {
   c
}
[download]

having the while accept an empty condition simplifies the translation.

By the way, C doesn't allow an empty while condition:
a.cpp:5: ANSI C++ forbids an empty condition for `while'