How often does that happen? The way I look at it, the bank gave you the CL for a reason...to use it. Obviously to pay it back but to make use of it.
It is unsecured, borrowed money, plain and simple. Unless someone's credit or financial picture drastically changes or appears to be an adverse risk to the lender, they are within their rights to CLD, close or demand full payment on that CC. It is an established T&C contract between both parties when applying. Obviously, if one is dilligent, pays as agreed and is within the T&C without any issues, then there really shouldn't be much of a concern.
DTI is all monthly debt payments. CCs, student loans, mortgages, etc.
Unsurprisingly, wikipedia actually explains it quite well. http://en.wikipedia.org/wiki/Debt-to-income_ratio
Loans are your monthly payments. You DTI would be ridiculus if it was your total loan amount of something like a mortgage. I believe CC debt is calculated by minimum monthly payment.