No. Satoshi did not make his invention a secret. He made it public beforehand. At the moment of its establishment the bitcoin had been worth nothing. There were no proof it would worth something in the future. There were some people who believed it will worth and they started mining. Why do you think that the risk they took should not be covered now?
About the difficulty at the beginning: the first miners were very slow. It was just straight implementation of double sha256 in pure C code without any optimizations using mmx, sse, GPU or whatever. Still the mining had been producing 50 coins a block regardless of GPU mining introduction, FPGA or whatever. Yes, a year ago the bonus had been halved - just as per design.