bash leaks sh-np-NNN files and pipes in /tmp/ when command substitution is used

[Yuri]

Some of my scripts use command substitution, and now I see that there 
are lots of files like these in /tmp:
prw-------   1 yuri   wheel          0 Dec 10 13:32 sh-np-1386738492
-rw-r--r--   1 yuri   wheel    3278909 Dec 10 14:54 sh-np-1386721176

Besides the obvious question why they aren't deleted in the end, two 
questions:
Shouldn't bash delete the pipe files once the child process opened them? 
On most platforms the deleted pipe will function the same with the 
benefit that it will get deleted automatically.
Why some of he files /tmp/sh-np-NNN are regular files, not pipes? When I 
look through code I see mkfifo call creates them. Shouldn't it always 
create fifos and not files?

bash-4.2.45
FreeBSD-9.2

Yuri