They can and do - it is a principle called "acceptance by conduct" - check my own
threads on how to combat this.
In a nutshell they are giving you 28/30 days notice of the change to the terms and conditions. You can write back and say that you do not accept this amendment and see what happens - it is likely that they will say that they no longer wish to do business with you.
However, to do nothing, but to continue using your account in a normal fashion, would indicate acceptance on your part. You do not have to accept in writing, merely continuing the arrangment is sufficient enough.
The alternative is to send an alternative amended term yourself, with a notice period of 28 days, and in your favour. Again, if they do not accept in writing, but continue operating the account as normal then they will have been deemed to have accepted the amended term...