Even if the various network address translation (NAT) functions, firewalls, wishy washy permissions and local-network-sharing restrictions don't stop the probing and hacking, they could use the connection as a private proxy server to mask activity for which you'd be blamed, or just brick your TV, STB or cable router just to cause trouble.
A negligible risk that will eventually become a certainty
That doesn’t mean it's going to happen. It's extremely unlikely, just as widespread hacks of photocopiers, printers and other office devices burdened by negligible intelligence never really materialized.
Some people have done it, I'm sure, possibly even using hackable printers as the only reasonable point of penetration through security that is otherwise tight as a drum.
Companies holding enough sensitive data to be worth the effort of squeezing into them through a printer probably already understand the risk (and failed to fix it or admit they haven't).
For nearly everyone else, a vulnerable printer or set-top box or Internet TV is so small a risk it's not worth even thinking about unless you've made your home and work networks so secure you have nothing to lie awake nights worrying about except things you're not sure would work anyway.
That could change if bits of malware suddenly appeared that were able to infect and take control of TVs or STBs or home-theater-system controllers or any other of the growing number of other Internet-connected appliances.
Think of the mayhem, the suffering you'd have to endure if some soulless hacker in Ukraine were able to pwn your dishwasher and demanded you pay 'protection' money to keep the water spots off your glassware.
The thing is, that's not even much of a joke. I don't personally see much point in wiring appliances to the Internet. But home networks, entertainment systems, storage devices, alarm systems, air-conditioning and heating systems, internally wired VoIP and all kinds of other smart home-based systems are becoming common enough to provide a tempting target, even if the payoff for pwning one hasn't really shown up yet.
At some point relatively soon, even non-geek consumer households will be packed with so many devices able or required to hook up to the Internet that they'll qualify as little enterprises themselves.
Design and manufacture of those devices, for the most part, will focus on aesthetics and use, not security or efficiency. Your STB, TV and home stereo will all be smart enough to be hacked and have network connections available so any laptop in the house can be used set the DVR, change channels or stream video.