Bro, this is your central argument. It hinges on the assumption that people know that on a 'normal day', 'a 90 min shoot is not $15'.
The problem is that you apply this to all companies equally. This is not true. Most consumers do in fact know that on a 'normal day' for Eu Yan Seng, Manhatten Fish Market, Dian Xiao Er, the price is not what Groupon offers. I 100% agree. So if you have this kind of brand, sure, consider Groupon.
However, how do they know what 'a 90 min shoot' is worth for a photographer they've never heard of? If a newbie photographer is charging, a 90 min shoot may not even be worth $15. If a pro is charging, then sure, it's probably normally far more than $15- but how would they know you're a pro, and what your 'normal day' rates are?
Besides, say they know your 'normal day' rates. What's stopping me from going to ANOTHER pro doing Groupon as well the next time I need a 90 min shoot? So each Pro does it only once, but visitors to Groupon get an endless stream of 'pro photographers' charging $15. In short, a 'normal price' for these customers are now... $15.
You see that with no-name salons or restaurants with no established price. Today I eat there on a Groupon deal, tomorrow I eat somewhere else, also on a Groupon deal. I understand they are all professional restaurants for whom a "normal day" is not this price; but I have the ability to choose from different professional restaurants everyday. Who earns from all this? Groupon.
But this is not even a no-name salon or restaurant, with significant cost overheads and fixed costs. You are an individual providing a service. They can't walk in, look at your establishment, and know that a ten-course dinner here surely couldn't cost $15. Instead, you are providing a service. Unless you are like a David Gan, for whom everybody knows how much your service is worth, unfortunately, customers who have never heard of you will just assume you are worth $15.
This is not 'lowballing'. It is simply saying, 'well, I have no way of telling how good you are... I can't look at the product, I can't look at your overheads, I can't look at the freshness of your ingredients vs other restaurants. You may have good gear, but so does the other pro down the street. You give me nice photos, but almost all wedding photos I see are great. I'm really not so discerning between photographers, unlike for restaurants. So how to tell how good you are? Well, since you charge $15, you are willing to take $15. You must be worth $15."
Again, this is not lowballing. It is taking your price as an indication of quality.