In Chapter 6 of To Kill a Mockingbird, why does Scout say that cards are fatal?

When Jem shows up before a gathering of the neighbors without his pants, Dill makes up the excuse that he lost his garment playing strip poker.  When Atticus, suspicious, asks if they were playing cards, Jem answers that they were only playing with matches.  Scout thinks, "matches (are) dangerous, but cards (are) fatal".  She means that to be caught gambling at all would invite punishment, but to be caught actually using cards would bring far more severe consequences.  To be playing with matches, even when an element of gambling is involved, indicates a certain naivete and lack of sophistication , but to be gambling with actual cards is the real thing.

The children are in this predicament because Jem and Dill had decided to trespass at the Radley place to look in the window in hopes of catching a glimpse of the infamous Boo.  Unfortunately for them, Mr. Nathan Radley saw them, and, not recognizing them as children, let off a shotgun blast to scare them off.  When the young interlopers tried to flee, Jem caught his pants on the fence, and wriggled out of them to escape.  When he, Scout, and Dill were confronted by the neighbors, Jem was embarrassingly pants-less, and had to quickly come up with an excuse for his situation.  It is then that Dill offered that Jem lost his pants playing strip poker (Chapter 6).