A potentially dangerous Request.Form value was detected from the client

I am getting the message: 
A potentially dangerous Request.Form value was detected from the client
when trying to save the read the value from a textarea which has the text ON=45 in it.  It seems that the letters ON and = are causing a problem?  If I put ON.=45 it is ok.  Can someone explain to me how this is considered a potentially dangerous value?  I'm trying to explain to my users why this would be bad, but I'm at a loss to explain.
Also, turning off the Validation for the page is not an option.
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