It might be because they are a stupid idea.
Anyway, Rule 153 of the Road Rules defines what a bike lane is. It says:
A "bicycle lane" is a marked lane, or the part of a marked lane:
(a) beginning at a bicycle lane sign applying to the lane; and
(b) ending at the nearest of the following:
(i) an end bicycle lane sign applying to the lane;
(ii) an intersection (unless the lane is at the unbroken side of the continuing road at a T-intersection or continued across the intersection by broken lines);
(iii) if the road ends at a dead end—the end of the road.
The "bicycle lane" sign looks like this:
and the "end bicycle lane" sign looks like this:
The problem seems to be the extra bit that says "between 4.00 and 6.00pm" or whatever time it says.
It is interesting that the default position seems to be that the poor, hard-done-by motorist gets the benefit of the doubt. The motorist would only have been issued the expiation notice if they blocked the lane between the designated hours. They chose to and should be fined.
I would have thought if the extra sign saying when the bike lane is in operation is the problem, the result would be that that part of the sign can safely be ignored and the bike lane is in fact in force all day - as it should be.
Nobody would suffer because nobody has been issued a fine outside of the designated hours. It just seems yet again a strange result that a person can park their car in a clearly signposted bike lane, thereby blocking and endangering cyclists, and get away with it for what is a minor technicality.
If this is a problem, you would hope it would be sorted quickly. It is a Regulation so does not require a new Bill to go through both Houses of Parliament. It could conceivably be done overnight. I won't hold my breath though.