Not true, several reasons for calendars to trade at zero. I was trading CNSB 85 call line for a credit yesterday and it was also offered in the COB for a small credit as well for some time before it traded. This is a takeover stock, with an all cash offer so very little risk buying a time spread for 0, but depending on how a takeover is structured there could be a substantial risk being long a time spread it you don't exercise your calls at the right time.
There also may be some risk in buying a call time spread in a hard to borrow stock. If you are assigned on your short calls, your clearing firm might auto liquidate your position for you.
What company is CNSB? I can't find that ticker on Bloomberg. Do you mind showing these markets that are offering same strike calendars for negative value.
A longer dated American option should always trade at a greater price than a shorter dated because if you had to choose between them for the same price you would always pick the one that has more time to payoff.