for *BSD bsdtar command option u = r = append, that is can't update file
I think they both append,
man tar on Linux says
append files to the end of an archive
only append files that are newer than the existing in archive
Here's a test to prove this
$ uname -s
$ tar --version
tar (GNU tar) 1.15.1
$ echo aaa > a.txt
$ echo bbb > b.txt
$ echo ccc > c.txt
$ tar cvf x.tar *txt
$ echo BBB > b.txt
$ tar -uvf x.tar b.txt
$ tar tvf x.tar
-rw-rw-r-- ian/ian 4 2012-11-08 16:43:10 a.txt
-rw-rw-r-- ian/ian 4 2012-11-08 16:43:17 b.txt
-rw-rw-r-- ian/ian 4 2012-11-08 16:43:26 c.txt
-rw-rw-r-- ian/ian 4 2012-11-08 16:48:22 b.txt
Note that the tar file now contains both old and new versions of b.txt
OK so on to the nub of your query
why tar command in Linux can update file in tar archive,but bsdtar can't.
You can install Gnu tar in BSD in
See Package Information for gtar-1.21-static.tgz (sh)
Why can't tar update in-place?
random access vs serial media
The name "tar" is a contraction of "tape archive". The command is intended to be usable with archives stored on serial media such as tape. The tape drive cannot insert new lengths of tape into the middle of your tape, nor can it cut out existing sections of tape. Nor can it read at the same time as writing. To transform a tape with contents a.txt, b.txt, c.txt to one with a bigger b.txt, first the whole tape must be read, the contents stored (in memory or on disk) and then the tape rewound and the new contents written in full. It is faster to seek the end-of-tape mark and append the new version of b.txt.
Also, many tape drives (e.g. DAT) automatically add an end of tape mark at the end of each write operation and this cannot be prevented. Therefore rewriting even a same-size or smaller b.txt at it's existing position would leave c.txt permanently inaccessible.
It isn't possible to replace content in the middle of a file with content that is of a different length. To replace a file containing "bbb" with file containing "bbbbbb" you must read the old file and write a new file. Therefore it is still easier (and may be considerably faster) to append "bbbbbb" (plus file metadata header) to the end.