The benefits you are referring to are not provided by Starwood Preferred Guest. They are provided by the card provider. Ameniti, from United, for example, also provides its members with the exact same benefits you have just described: free continental breakfast, an additional welcome amenity etc. However, the trick is:
YOU MUST BOOK THE RESERVATION WITH THAT PARTICULAR COMPANY.

So, with Ameniti, you must book through ameniti, and with Amex, you must book via Amex with different rates than what SPG.com is offering therefore excluding you from any SPG promotions or even sometimes points and stay credit. Also, 99 percent of the times, these special package rates offered are significantly higher than SPG's best available rate. I recall attempting to make a reservation via Ameniti for W Seattle. Upon inquiring, I was told a king wonderful room would have a rate of 199 per night, which was 30 higher than spg.com. I then inquired about what kind of breakfast and amenity I'd get. The agent informed me that in order to receive breakfast and extra amenity, I'd have to book a different rate plan - for $239. So basically, you get complimentary breakfast and amenity for $40

Those offers sound great from the surface, but you will soon discover that no other source will offer you more amenities than what SPG Platinum from the actual Starwood Preferred Guest will offer you