A few of the nation’s biggest wireless carriers have wised up. They are putting a stop to apps that let Google Android users tether their smartphones for use as modems without paying the carriers’ extra fee.

Free Android phone tethering apps that are typically found in the Android Market are not available for Android phones on AT&T, Verizon Wireless, and T-Mobile USA. Sprint customers apparently are still able to access many of these tethering apps from the Android Market.

AT&T and Verizon have said that Google chooses which apps are in the Android Market. Neither company would comment on whether they asked Google to remove the Wireless Tether app or any other free tethering app from the Android Market.

However a Google spokesman told Fierce Wireless that it is not blocking the app. Instead it’s simply making it unavailable for download on certain carrier networks at the request of those carriers.

In essence, the apps are still on the Google Market, but they are just not visible to users on certain carrier networks. Apps are only hidden from view on certain carriers.

If your sly enough you can you can still side-loaded these apps onto the device. If the developer distributes the application file in a way other than the Android Market (say, just as a download from a Web site) a user can install that on his device.

AT&T has been sending e-mails warning users who are tethering their phones without paying the extra fee. AT&T charges $20 extra per month for the tethering feature. Customers who tether are given 4GB of data to use during the month. Customers who exceed that limit are charged $10 a gigabyte thereafter.

T-Mobile USA has the best value at $15 a month in addition to a smartphone data plan for 5GB worth of data per month. And Verizon Wireless charges $20 a month on top of its $30 smartphone data fee to use up to 2GB of data per month.