They don't have to be corrupt at all, just the simple fact that media and lobbying power are mostly controlled by capital expended it would be naive to assume that non-corrupt politicians can not be influenced.
If they could not then there would not be a lobby, and media would not be as politicized as they are.
In my opinion, a politician has some freedom to choose who they listen to, and politicians who predicate that choice on their personal enrichment are corrupt.
But that's a black-and-white view. The 'gray' version is that it is perfectly possible for a politician to be of good intentions, but presented with a choice of 99 parties funded by special interests with megaphones and one lonely voter you can't fault them for having more - and usually better argued - input from special interests.
That's why many countries forbid lobbying entirely (not that that's 100% effective, but it's a start).
If they could not then there would not be a lobby, and media would not be as politicized as they are.