In many cases, IBM will
sell you hardware and software. They will help you install and set it up, but at the end of the process
you own your data. It's not on some 'cloud' controlled by IBM, it's on a rack of IBM-branded servers in your own datacenter or server closet. The software is almost invariably some version of Linux, so you can audit it for security.
Your question is like asking, "How can I trust that Western Digital won't turn over the data on my hard drive they sold me?"
I suppose it's possible future revelations will show that all major software and hardware packages supported by US firms contain legally-mandated secret backdoors, but we're not that far down the rabbit hole yet.