"weapon of choice" doesn't necessarily mean that Murray actually uses it to play tournament matches with.
It could just mean that if you asked Murray which of these 5 weapons you should choose, he would reply - "choose this one".
Those are some real verbal gymnastics. If Head would have to go that far, adding facts that were not there, to make the statement seem plausible/defensible, then their collective noses are growing.
Clearly, the implication of referring to a racquet as his “weapon of choice” is that it is the racquet that he uses. Players could “endorse” racquets without going so far as to create the false impression that they actually play tournaments with the racquet.
Some degree of exaggeration is even allowed, in any event, as alternative medicines make all sorts of claims and they are legally sold.
There is no real difference in the case of a modified as opposed to a completely different racquet as they still rely on the 'exaggeration' that the players uses the exact same one you will buy.
Alternative medicines often make claims about the benefits, such as they will make prettier, happier, blah, blah, blah… Similarly, racquet companies make wild claims about the benefits of their latest, greatest technology. These claims are a matter of opinion, so the person making the claim can get away with it, usually anyway. However, the alternative medicine company could not make a false statement of fact, such as to the ingredients of the medicine. Similarly, if a racquet company makes a statement that a pro plays with a racquet, when he does not, this is not “exaggeration” or puffing, it is a false statement of fact (i.e. lying). On the other hand, I would not consider modifying a racquet, such as by adding lead tape, to even fall in the category of an exaggeration.
Also, the question is not whether it is legal or not. This message board is the court of public opinion. We should expect more from businesses. Maybe you don’t, and that is okay too. You would probably say that the world has bigger problems, and I would agree. All things being equal, however, I would prefer not being lied to.