Widow is a word that has a specific meaning, a meaning that cannot be allowed to change. Think of it like the word "woman".
Surely trans-widow and trans-woman are very similar if opposite terms. Both adopt an existing word with a very specific meaning (adult human female and wife whose husband has died, respectively) and added another word to it in order to suggest the comparison with the underlying word, whilst fully acknowledging that neither are the actual word because the entire meaning has been changed by the word added on the front. The former is not an actual widow nor is the latter an actual women, because if they were the "trans" word wouldn't be there.
A horse is a four legged, large mammal you can ride - a seahorse is not a horse at all, but it incorporates the word "horse" because of the superficial similarity, but the word sea is what completely changes the meaning and makes absolutely clear that we are not talking about horses at all.
Actually, thinking about it, it's a bit like trans-gender as well.
Gender is a series of sex based stereotypes and the related issues. Trans-gender is comparable to gender (the same stereotypes), but is actually very different. The addition of the word trans makes it clear that stereotypes are no longer sex-based (ie in the word trans-gender, gender doesn't literally mean the same thing as when gender is used alone, as the stereotypes are being attached to the opposite sex not the stereotypical sex as you would expect when using the word "gender")