The reason for this rule is to protect the forum from a sort of malicious behaviour known as "flooding".
Flooding is when someone using a special script automatically bombards the forum by posting similar messages. The messages actually flood the server so fast, that they use all its bandwidth...
Plus, there's a HUGE pile of messages to clean up...

Simple enough.
The 60 secs rule is actually a VERY nice thing. I'll be surprised if such forum doesn't have it on...
Ivan