But that's not a very compelling argument on its own, since the Ethernet link is just one link in the chain. Having a gigabit port doesn't help much if the TV can't handle decoding video at those bitrates in real time. It's definitely possible that TV manufacturers choose 100Mbps ports because they know the TV can't deal with huge streams for other reasons.
It's an interesting situation for the manufacturers. Even if 99.9% of buyers will never see streams above 100Mbps, and even if that other 0.1% can't effectively use them, it might be worth it to bump the port to gigabit since complaints about 100Mbps ports come up so often in reviews and in online discussions. Maybe throwing in a borderline useless gigabit port would generate enough sales to justify the marginal BOM cost increase.