Er, no. A trans man is a man. That's why it's called a Trans MAN. And in fact, that's part of the problem; having women play trans men and men play trans women just reinforces the stereotype that trans people are just cross-dressing or playing dress-up, that they're not ACTUALLY men and women but just "a woman in flannel" or "a guy in a dress." So that's not helpful either.
(not to mention that, in my opinion at least, I wouldn't doubt that casting directors do that because they want the character to be "obviously" trans. So if a trans person auditions for the role but doesn't actually look like a man in a dress but looks like any cis woman, the director is gonna think "this won't be believable!" because they stereotype (or think the audience stereotypes) that all trans women are square-jawed five o'clock shadow rectangular men in dresses, and won't believe that this person is actually a trans character. Despite being played by a trans person. Same stereotyping applies to trans men too.)
And I mean, it'd be great if your trans or nottrans status didn't matter for getting roles. But like I said, if you're a trans actor, you're very VERY lucky to get cast in non-trans roles; you're typecast already just by being trans. So having a cis person take a trans role is taking the only roles that anyone's actually letting you have away from you. That's why it's a dick move.
E:
Got it, what were the roles? I feel like I'm missing out here. Thank you for educationing.
Tambor won the Golden Globe and Emmy for his role in Transparent. Huffman was nominated for an Oscar for Transamerica, and Leto won for his role in Dallas Buyers Club.