They don't just want your fare, of course. As soon as you're a semi-captive audience they pitch "just one quick stop" (their cousin's trinket store), or quickly slide down the Maslow's Thai Tourist hierarchy of offerings: You want Shopping? Thai massage? Girls? and, if they're very desperate...Boys?
Today my friend Ben and I wanted to cab it to Pratunam Market, and were having a hell of a time. The taxi mafia boss who stands permanently outside the hotel told us he would take us in a metered cab, only to close the door and breathlessly get out his brochures of "best quality" (i.e. knock-off) Louis Vuitton. The next cab told us he had a meter, then as we sat down refused to use it, forcing us to climb out of a moving cab (moving at 2mph, but still) and the next guy just flat refused.
We tried a different tactic - perhaps this is an obvious one and I'm just slow. Instead of getting in the cab and asking for Pratunam Market, this immediately marking us out as rabid consumers willing to field any sales pitch the driver has in his pocket, we asked for a nearby hotel. Ah, now our buying habits are concealed beyond the wit of our potential driver! Would you know it, the very next cabbie took us in, turned on his meter and didn't try to sell us anything. In return, he received a handsome tip, proving that it pays just to shut the flip up sometimes.