This is not true. He said he will put 2500USD on the table for the best idea. This is not "for free".
Why don't people just post their ideas with some sort of disclaimer that the 2500USD have to be paid before the idea is adopted?

Many people just don't get the point that an escrow is absolutely useless. The OP could very well put the 2500USD to an escrow,
let you post your ideas and then ask the escrow for a refund because none of the submissions meet his definition of "a good idea".

The escrow is a misconception, does not give you any additional security, and is - in the way it is used here in the forum - completely pointless.
Why do people still do it?