Because she can get pregnant!!!! Women with the syndrome she is "reported" to be diagnosed with can still get pregnant. It may just be harder for them. There are different degrees of the syndrome. They may not ovulate at all, making it impossible to get pregnant, or they may ovulate less often. Making it a little harder and take a while longer to hit a cycle where they ovulated. Apparently, Kate's syndrome is not that bad (if she has it at all - remember she was nurse, and knows the answers to give to questions to get a fertility doctor to treat you) Rumor is she pretty much lied the first go around about how long she had been trying. She had always wanted twins and was over-interested in multiples and the attention that the Macauheys (sp?) received. She wanted multiples. She did not have the eggs fertilized outside of her body and then have the embryos implanted, she took a drug that stimulates egg production. Then she got prego the old fashioned way. She became pregant immediately with the twins, and the doctor told her the second time she showed up , that no, she didn't need the drug, she would be fine on her own, and the chance of multiples was too high. So then she went doctor shopping. Found a doctor that must not be that ethical, and received the same type of drug. She was hospitalized after this for over-stimulated ovaries (probably because healthy ovaries will react this way when given this drug), which means that she was producing way too many eggs. They were told NOT to have sex, to wait until the next cycle, that having sex would result in a large number of multiples, and guess what she ran home and did?
So THAT is why Jon would get snipped. Because Kate CAN get pregnant. She is just such a control freak, she couldn't leave anything to chance and wanted to be pregnant immediately and wanted to control the number of babies.