To my understanding, the original purpose of trademarks was to protect the buyer. If I buy a bicycle listed as being from X, I can expect that it was manufactured by X, or at the very least endorsed by X (e.g. Kirkland products). If a different manufacturer Y labels their product as X, then I no longer have that certainty.

But corporations have taken that and gone way too far on it. If I describe a product as being "compatible with X" or "fits on an X", that in no way makes a claim that it is manufactured by X. Like how tv manufacturers should be able to say "Perfect for watching the Super Bowl this weekend.", but avoid doing so for fear of being sued. There's no endorsement at all there, nor any dilution of the trademark, and yet it gets treated as though the words themselves are protected.

I believe it is an implicit "you make them look shoddy" if your product doesn't work after they change something. Self-produced ones at least they can check for backward compatibility but they have no way of guaranteeing any of their changes would break any fly-by-night or obscure adaptors.

Rather overkill in practice for a legal doctrine. But I can see their concern, and why a company would dislike it over the sheer tech support call volume alone. Their first response being "stop it!" makes sense in that light.

Open standards are a good way to prevent issues while keeping both sides happy (notably it also keeps company names out of it except in deniablenways such as say listing GMail as an example of a POP3 user - it doesn't equate the two). Open standards aren't automatic or free though and there may easily be gaps because they never thought to specify a given portion for interoperability.