In short, what you're really arguing is why isn't science as rigid as religion. It isn't because it's not based on dogma.First, the database update was no secret. NOAA previewed the changes years in advance by publishing descriptions of its methods in peer-reviewed scientific journals. The government agency also announced the new data set through public statements, and created a tool for users to compare and contrast temperatures from before and after the update. NOAA also makes its data and computer code available for anyone who wants to check the numbers. The new data set is called nClimDiv, and you can find more information about it on NOAA's National Climatic Data Center website.
Second, NOAA never changes the actual temperatures that were so carefully recorded over the decades. But it's no simple task to compare the present with the past. Methods of measuring temperature have changed markedly over the past century. The database tweaks are meant to make the comparisons between modern and obsolete technology more accurate.