It should be obvious to anyone that RedHat has a vested interest in making the vast majority of Linux distributions dependent on technology it controls. Linux is its bread-and-butter.
It appears RedHat has realised that, through systemd, it can readily provide preferential support for its own projects, and place roadblocks up for projects it does not control, thus extending its influence broadly and quickly. By using tenuous dependencies amongst its own projects it can speed adoption even faster.
Once it has significant influence, and the maintainers of competing projects have drifted away either out of frustration or because they are starved of oxygen, RedHat knows that they can effectively take Linux closed-source by restricting access to documentation and fighting changes that are not in their own best interests.
At this point, they can market themselves as the only rational choice for corporate Linux support -- and this would be perfectly reasonable because they would have effective control of the ecosystem.
Linux (as in a full OS implementation) is an extremely complex beast and you can't just "fork it" and start your own 'distro' from scratch anymore -- you would have to leverage a small army to do it, then keep that army to maintain it. It's just not practical.
At the same time, Linux has matured to the point of attaining some measure of corporate credibility, and from RedHat's point of view, it no longer needs its 'open source' roots to remain viable. RedHat also, understandably, fears potential competition.
Through systemd and subsequent takeovers of other ecosystem components, RedHat can leverage its own position while stifling potential competition -- this is a best-case scenario for any corporation. It will have an advantage in the marketplace, potential customers will recognise that advantage, and buy its products and support contracts.
I hope you can understand why many see this as an extremely compelling case. Arguing that RedHat has 'ethics' and would 'never do such a thing' is immature and silly -- RedHat is a corporation, it exists to profit from its opportunities, just like any other company. To attempt to argue that it would not do so is contrary to what we can assume is its default state.
It's no 'conspiracy theory' to assume that a corporation will behave like a corporation; arguing that it is just makes one look like a naive child. systemd is one large step toward RedHat gaining the ability to reap what it has sewn -- for its benefit and not necessarily ours.