> Could you explain why gmake deviates from make in such an important point?

Is it important though?  What makefiles really care?  If it were critical then there would certainly be a lot of people upset about this but it seems like it only matters for the letter of the standard, not for any practical use.

The reason GNU make handles backslash/newline this way is lost in the mists of time so I cannot help you understand why it was done.  Most likely Richard and Roland just thought automatically condensing newlines was better / more user-friendly.  Their goal (AIUI) was not to perfectly emulate any other implementation of make: they wanted to create a make that worked best for them and their users.

Once the choice was made, and was in effect for a lot of years, there were potentially a lot of makefiles that assumed that whitespace was condensed (when showing output etc.) and it didn't seem important enough an issue to break backward-compatibility so when the POSIX discrepancy was pointed out I fixed it by changing it for .POSIX only.

The way it should have worked is to replace backslash/newline with ZERO characters.  That would have actually been a useful distinction because it would have allowed concatenation of text across lines (for formatting purposes in the makefile) without forcing spaces in them.  If you wanted a space you'd just include one before the backslash or after the newline, which almost everyone does anyway.  Once the opportunity to do it the right way was lost it seems fairly arbitrary whether you use one space per newline or collapse them into a single space.  In fact, the ONLY request I consistently get regarding handling of backslash/newline is how to avoid putting any spaces at all.  No one seems to care whether you get one space per newline or not.

> I believe that .POSIX should be reserved to cases where the POSIX standard does not agree with the behavior of the historic UNIX make.

I disagree with this interpretation of the purpose of .POSIX and see no justification for such an interpretation in the standard.  Its purpose is exactly what GNU make uses it for: to ensure that makefiles written to conform to the POSIX standard will work properly when run with a given make implementation.  Nothing more nor less.