Why? Because it's not Tendai (institutionally, practically, or doctrinally) and it's not Mikkyo without the proper transmission. What this group is calling "mikkyo" really has more to do with a redacted form of Shaktism (kundalini, shaktipat). I'm not saying that's bad (it's not for me but who cares about me?), merely that it's neither Mikkyo nor Tendai.
Really, the only way to learn Mikkyo in Tendai outside of the traditional Japanese system is through the Tendai Buddhist Institute (see link below), which involves very specific training and certain commitments.http://www.tendai.org/index.php?id=46
I can't speak for Shingon-shu on this question of how mikkyo is taught. My understanding is that your learning would involve a trip to Koya-san and a lot of language lessons. Someone else is going to have to explain how that system works.
If this group removed the words "Tendai" and "Mikkyo" from their promotional materials and other writings, and explained in one way or another how their teaching is Buddhist if they want to be understood as a Buddhist group, then I'd have no objections to their business at all.
Great River Tendai Sangha: a Tendai Buddhist community in Alexandria, Virginia, USA (near Washington, DC):