This is mostly hearsay from back in the day, but under the legal cloud that ensued, I doubt we'll ever get a real answer. As far I know, when Battletech was originally made and went to press the creators didn't hold any rights to the designs they used. Maybe originally thinking, "Who's gonna notice us?" or the Tommy Yune-like, "Foreign copyrights don't apply to us!" Once it became apparent that Battletech was going to reach the size where it couldn't blatently flaunt the law (ie the game gets popular + Harmony Gold releases Robotech on TV) they scrambled to aquire the rights, which led to some (rumored) deal with Revel.
Remember, this was back in the days when, in all honesty, Japanese animation was so far underground that most people would have really not noticed the designs were stolen. In a side note, the origianl battletech book I had obviously had no artist on the team (at least not one worth their salt) as the mechs were always drawn in their line-art pose (be it mirrored or however slightly adjusted). Infact, although I posses the book no longer, I remember a "picture" of a swamp battle between a line-art Macross mecha and some elementy school kid's robot arm.
Although I played it, I've always looked down on battletech for this. I mean, not so much for stealing the designs, but, if any of them were anime fans (and someone would have had to have been to get the line-art) why did they end up turning otherwise nimble mecha into slow-ass lumbering mechs? And who was the idiot that couldn't line up game stat weapons with the line art? Questions, questions...