My point is that if you call a Bangkok hotel or central reservation office and tell them " I saw this corporate rate/ internet rate (2000BHT+ for Shangri La's internet rate; appr. 5000+ BHT for Peninsula is a normal corporate rate), they will give it to you subject to availability after little persistance of yours and calling during the time when a real officer, not receptionist, answers the call. The booking sites are only a guideline for Bkk. But depending on the season/ some events, they might not have availability.
In a symilar situation a Hong Kong hotel removed the internet rate while I was on the phone with them once, so Bangkok is a big exception. All hotels are prety flexible with bookings. They might not even ask for credit card, and usually get just credit card and 24 hour notice for cancelations is fine.