bestbuy consumer credit card maintaining balance and promotional offers but just my limits for credit card which i dont even need to use Internet

 iam best buys loyal customer since 2008 as of 2010. i found best buy offers promotional offers and i decided to use that offers n buy electronics device which i need, and i bought tv,mac,slr and laptops etc items thru hsbc and now its capital one best buy credit card . i have been always good on payment even better but i was socked when  i found my limit was decreased 3600 to 2005 dollars which is not even m using n even if you all wants to see my payment histery was very clear

not even late or less amount payments and i called 1800 no and they said something is diffrents story to decreased my limits so i just paid whatever i paid n i decided not use this service and store any more coz its unfair decision which is decresed my credit limit whith out my discuss . and funny thigs they said was they send a letter some day on septmber or whatever which i never look coz i used online payment options so i dont need to look at it .if iam maintaining my paymets and credit there is no reason to decrease my limits 3600 to 2005 dollars ?????