Whenever I order a hamburger at a diner or TGIF type of place and specify mayo, they always put it on the side. If I order ketchup or mustard, they put it on the burger.
Why?
Some do. I guess yours are different.
This is good, not bad! Now, I can put on exactly as much as I want: just a little dab, or a great big smear. I can put it where I want: on the patty, or on the bun, or on the lettuce. I can put it both on top and on the bottom. I can do it my way!
Same reason they put salt and pepper shakers on the table, rather than salting and peppering your food for you.
Geez. Most folks would complain if they weren’t given a choice, but to complain about being given one?
We need to swap worlds. In mine, everything is always smothered in mayo, sometimes even when I’ve specified “No mayo”. I hate the stuff.
If it’s an extra request and not something that normally comes with a sandwich, I would expect it on the side at a regular restaurant, and on the sandwich at a fast-food type place.
They’re tired of having it sent back?
Who’s complaining? Geez.
If this is indeed the reason, why don’t they give me the same option with ketchup or mustard?
They’re just trying to clue you in to the fact that mayo on burgers is really fucking disgusting.
Apologies; I guess I was projecting more than I realized.
Some places do. And I love 'em for it! My faves are the “burger bars” like salad bars, where you can mix up the toppings however you want. Lots of lettuce, or none. Dill relish or dill slices or sweet pickle slices. Tomato slices, or wedges, or chopped. Jalapeno cheese! Oh, man…
Now, me, I love mayo on purt’ near anything. I also love leetle salty fishes on pizza!
(I hate this! I’ve only got microwave slop to look forward to tonight!)
Homemade mayonnaise used to be very dangerous if it got warm and sat out, so it’s kind of one of those cultural things that you don’t let mayonnaise get warm or people will get sick.
No worries. 
jsgoddess, this was my idea also, despite the fact that it doesn’t make much sense for the restaurant to do this. The mayo will end up on the burger anyway, but will be there just a few minutes less.
And to you naysayers, I say a burger without mayo is a good meal, wasted.
If you’re projecting after a hamburger with mayonnaise, you should maybe call the health department. Or a doctor.
Or an exorcist.
If they put it on the burger rather than the whopping tablespoon they put in a baby ramekin, they wouldn’t be able to charge you an extra $.75.
I’m looking at you, Applebee’s.
Or the Adelphian Lodge.
(Possibly NSFW. Also possibly not safe for human sanity…)
(The National Lampoon, “Back when it used to be funny” TM)
Generally speaking people east of the Pacific, south of Oregon, north of Mexico and west of Nevada like mayo on their burgers. I am one of them. Most of the rest of the world, with individual exceptions, think mayo on a burger is an abomination. Go figure. IIRC Europeans in the early 80s (I haven’t been since) considered mayo disgusting on anything.
Mayo is disgusting? It’s just eggs, oil, and vinegar or lemon juice. I guess there are some who legitimately dislike mayo. Most of the people I’ve known who’ve disliked mayo have actually never eaten it, though. They’ve only had miracle whip (which tastes like seagull shit and ogre smegma).
Maybe there’s a practical reason: It’s easy to put catsup and mustard into squeeze bottles, so squirting those condiments onto your sandwich doesn’t require contact between the container and your cooked food. However, mayonnaise would typically be kept in a tub in the kitchen, and be dispensed with a knife or spreader. Putting the mayo directly on your bread or meat would either require a new spreader each time, or create cross contamination (and yucky crumbs in the mayo.) Putting a dollop of mayo into a clean bowl or cup is easier and more sanitary.
Generally, I’m curious about how Dopers come by their vast knowledge. But in this case…