Welcome to Android Central, it's great to have you here. I'm no expert on tethering but that sounds nefariously like a carrier limitation, but that could just be my tin-foil hat at play.
Like pdanet+ app once you go pro you can use unlimited tethering and the data that you use via computer appears as phone data usage not comp usage, making you hidden from your carrier that you are tethering.
Have you taken a look at other tether apps available in the Play Store? I agree that it's a limitation with the App wanting a purchase or some sort of paid upgrade.