It's standard in all IT contracts. You will not get a job otherwise. The point isn't that they take everything, but you'd have to run it by them for approval. You are not then not allowed to have e.g. a second job with a potential competitor or provide the same service as a freelancer as through your company.

In this case, I assume IBM wants its employees contributions to be noted as open source support. They are a major contributor to Linux, in and off paid hours, they will not have a problem with one employee doing open source work, just want the PR.