Could it be more restricted, I can't get access to .c member.
Why doesn it allowed, Tomasz?

Why do you think it should be allowed? You have redefined it, and there is no .c in the new version of your structure.

I mean, I don't know about how redefinition works, In Masm it does error (when define with difference member set). so Masm is more restricted.

Would be nice if Fasm is restricting doing this (becuase of each include files may have the same name with difference set of members and user don't know)

Oh my eng.

Many of fasm's macro tricks are based on the feature of redefinition, which is allowed for all kinds of macros (so struct macros too) and symbolic constants. So such restriction would just bring down all the power of fasm's macro system.
But, on the other hand, you can also utilize that power to achieve what you need - you can define macros for structure definitions (like the ones I have posted here some time ago) and build such restriction into them.