this makes me wonder if the 300 firmware is actually older or behind the prime one. on the last update for prime, over a month ago, this fix wasn't even needed anymore. You can tether normally now. the update erased all the old files and replaced with new ones allowing tether. I guess you all will get the update to allow tethering soon enough.
I'm sure alot would appreciate this fix until the next update for 300 allows it.
Motorola Atrix-Rooted on froyo 2.2.2 (Retired)
IPAD- 32GB WiFi ver. 4.2.1
Transformer Prime 32GB(Retired)
NEXUS 7 16gb Unlocked & Rooted JB 4.2.2 Twrp. TEAM EOS4
Galaxy Note 10.1 32gb- Stock JB 4.1.2
Samsung Galaxy SIII - Phablet
Prime/Tegra3 Gamers Thread