In some cases you could be right, however all of our girls accept credit cards. When a card is processed the girl is paid via check so whether she is public or not the money is traceable. The girls are 1099'd at the end of the year unless they are paid via a corporation they own.
It is an interesting theory for those non-agency, cash only UTR ladies.

Your explaination could be one reason, but there are many other reasons a girl wants to be UTR.

Contract girls from Vivid and Wicked are "discouraged" from escorting.  I'm sure the companies know it goes on, but they'd prefer their stars to be subtle about it.  Lately however, with the business being in the shitter, you now see a few contract girls openly available.

Same with Playboy magazine.

Plus there are others in the industry, including some pornstars, who looks down on a PSE, and may refuse to work with them.  Most talent has a "Will Not Work With" list of people they won't shoot a scene with.  When Sunset Thomas was the first pornstar to work openly at the Bunny Ranch she initially got a lot of flack from her fellow pornstars (Until they saw how much money she made - then they were lining up at the Ranch).

Also the girl might have family that she's concerned about finding out.  I know it may seem a small distinction, but I think most relatives would be more likely to accept her being a pornstar than a "prostitute", as they'd likely call it.