http://www.featureshoot.com/2015/12/harrowing-photographs-capture-the-gravity-of-the-hiv-aids-epidemic-in-tijuana/

HIV exists in every major city on Earth.  Wear a condom.  Don't share needles.  TJ is no different.  

I could be wrong but a lot of providers from San Diego don't like Tijuana as it's so close and it's a better cost for service and the lady's are just as good if not better.  

As stated above, US providers HATE Tijuana.  The providers there deliver a true GFE for way less.  On top of that there is a lot of racism in the US and it is common to find racists making negative comments--that usually aren't true--about Mexico.

You are not wrong, when I first started the agency, I advertised at CL San Diego, my posts would get flagged and San Diego providers would post about how dangerous Tijuana was among other things, whatever it took to scare potential customers from crossing into Mexico.

Why don't more US providers escape law enforcement and advertise in Tijuana and take first time clients?  That would make such a great screening process for them.  Hell you don't need code words like roses or donations.....You can even say what is on your menu

Because everybody knows as soon as you cross into Mexico the cartel is waiting to shoot you (sarcasm).

I agree with the scare tactics  because of what the media hypes that happens in Mexico. I live in both Otay Ranch eastlake and Playas de TJ. I have witness crime in Eastlake from shootouts at Apple store to home invasion  robberies to suicide all in the suburbs or Eastlake. But the crime in TJ is minimum.  
Now to prostitution in US to to her parts in the world I see the bad wrap tj thailand and Singapore get for legal prostitution. All the places I listed for example have a health care system in place to help girls out and vaccinate them as well. There is no shame either where in US prostitution is done in secrecy and so when girls get robbed attacked raped or a disease they are scared to go to get the treatment they need.  
The money differential is best for US hobbyist and for TJ girls because they will get money that will help them live or drug habit or whatever. Plus the girls in TJ sell you the euphoria and fantasy lacking from the business in the US without payin an arm kin a leg for it. What I spend in TJ I get a GFE where I get laughed at in the states for.
so I the girls over in TJ get a bad wrap for the wrong reasons when the girls in the US have more at stack to lose.

I understand a brothel with a brick and mortar location would want to stay within regulations. Do web-based agencies like the ones that post on the board stay on top of health cards and checkups?

Highly doubtful.  

Some do. Some don't.

Not if the standard overnight rate is 300 dollars. Ladies might as well get a vanilla job if you're taking home 300 dollars for 24 hours of your time.

I don't think anyone is offering $300 for 24 hours.

What is going on is what is going on worldwide and that is a concerted effort at massive birth control by this scare campaign.  The chances of a man getting HIV from a woman during straight sex are slim and none, even if she happens to be infected.
  TJ has a huge gay male population that engages in all sorts of unhealthy sex practices and drug use.  I was walking through the Revolution area on the way to the zona about a month ago and had to detour around a huge gay pride parade with men dressed up as women in beauty queen outfits.  In fact Revolution has been completely taken over by the gay crowd.  I get so upset by the government crap that is always going down.