because she is poor and lives in the seam. this means that unless she wants to put her name down for 'The Hunger Games' an extra 3 times (1 time per family member) to pay for food, she needs to break the law to prevent starvation. As she lives in a world run by 'The Capitol' they have laws against poaching and leaving the district because they are corrupt and do not want people being happy after 'The Dark Days'. also it was something she did with her father when she was age 11 up so she grew up doing it. Her father was killed in a supposedly accidental mining accident we later find out to be set up by 'The Capitol' to rid district 12 of the rumours the miners were spreading
