Mexican restaurants are often family run, and as such rely on 'folk wisdom' for much of their activity.  Where artificial sweeteners are concerned, the average Mexican restaurant probably mistrusts all of them. Here's why.

Probably because they contain sugar, are treated with wood alcohol and are not really 'safe' for diabetics. 

From a website 

 http://jstevens.wordpress.com/2008/02/20/how-sucralose-aka-splenda-is-made-and-why-you-want-to-avoid-it/ 

  "when sucralose was shown to not raise blood sugars, it was the pure substance that was tested, not the mixture that is sold to the public.  Dextrose, sucrose, and/or maltodextrin are definitely going to raise a diabetic’s blood sugar.  There is also a great deal of evidence that artificial sweeteners actually cause an increase in appetite, causing people who consume them to take in more calories than they would otherwise."

  One small study of diabetic patients using the sweetener showed a statistically significant increase in glycosylated hemoglobin (Hba1C) 

As for equal, aspartame is also treated with chlorine, much like carrots, and the compound contains dextrose, primarily.  When aspartame reaches 86 degrees, it converts into formaldehyde, which then changes to formic acid; not a pleasant thought.  

Finally, you can check out this link,

 http://www.shirleys-wellness-cafe.com/nutraswt.htm 

to get an in depth view of the opposition to these two items.  The temperature problem alone should be enough to stop those from tropical origins from using aspartame, since 86 degrees is below the nighttime low temperature of some such locations.