It is undiscriminating or discriminating, but when you drop the -ing then you use indiscriminate. Don't know why. This is what we accept. 

Also, rules always take second place to common practice. Your English teacher would spit and moan, but it is the truth. Language experts in the distant past tried to destroy won't for the same reason they succeeded in getting rid of ain't - because it technically doesn't make sense. Do+not becomes doesn't, and is+not becomes isn't, but somehow will+not becomes won't and am+not becomes ain't. 

But practicality and common usage are always more important than rules. Ain't is easy to replace with isn't, but won't doesn't really have an alternative so we kept it. 

The most important part of language is effective communication, and the reason you need to know the rules is so you know how to be formal. If you consider yourself a writer, you also need to know the rules so you know how to break them effectively.