You're trying to pull a fast one on the kernel, and you lost. "standard output" is file descriptor 1. When you open a new file, it goes to the lowest unopened file descriptor. Normally, 0, 1, and 2 are all busy being standard input/output/error, but by closing STDOUT, you left room for another file to be fd 1. Hence, you lose. If you don't want that behavior, don't do that.

If you want to ensure that the child's standard output goes nowhere, open STDOUT onto /dev/null instead.