Many years ago I was walking down the street in my hometown on a week-day Yom Tov and passed our local kosher meat store. Outside I saw a very bewildered-looking young man (no yarmulke) peering through the window. When I asked him if I could help him he replied that he was a Muslim exchange student at the local university and got his meat at the kosher store because he could be sure it was halal, and that many of his friends looked for the kosher mark on food in the grocery before buying. I explained to him about the Yom Tov and told him when the store would be open again. He thanked me very sincerely and went his way.
Putting a sign in your store window that you are halal-compliant doesn’t mean that you endorse Islam. If it leads to extra parnassah for the Jewish owner why not? I’ve also know Christians who buy kosher because they think it’s better supervised and healthier. Incidentally, Islam and Christianity aren’t technically avodah zarah – they have other classifications (e.g. shituf for Christianity) and don’t fall under that strict category of issurim. Let’s worry about more important things – like being respectful of each other when we post to the Coffee Room.