Whoa, I think you're conflating webtoons and the site webtoons.com. You're right that the word webtoon is used to refer to a specific form of webcomics, and you're also right that the site webtoons.com is managed by Naver, but webtoon and the site webtoons.com are two separate things. One is a format that's not tied to Naver; the other is a website run by Naver.
Note that you don't have to upload your comic on their site in order to get hired. If you're uploading elsewhere and you're doing really well, they still might approach you for a deal.
Competition is definitely a factor behind the different atmospheres of webtoons.com and Tapastic. But the way I see it, it's generally not the creators themselves who get cutthroat-competitive. It's the readers doing all that downvoting and complaining. I think part of it is inevitable as the site becomes bigger, and attracts more consumers (who are not creators). The users treat the comics as a product they consume. They don't care about building a relationship with the creator. They just want a fun, convenient reading experience, and there is absolutely nothing wrong with that in and of itself.
Of course, it's more complicated than that, but that's one aspect that I think we all should pay attention to, including people who have no intention of posting their comic on that site. Not everyone wants to attract, or has to cater to, "consumer" readers, but we should at least acknowledge their existence and make our choices accordingly. Even if that acknowledgement is as simple as "yeah, there are a lot of people like that, and I am not catering to them," that's better than nothing.